At issue in this case is whether the interpretation of G. L. c. 32, § 4(l)(s), by the Contributory Retirement Appeal Board (CRAB) was a reasonable one. See Foresta v. Contributory Retirement Appeal Bd., 453 Mass. 669, 676 (2009). CRAB determined that Claire Barker had the required ten years of creditable service with the State employees’ retirement system (SERS) to establish contract service under G. L. c. 32, § 4(l)(s). The State Board of Retirement (board) considered CRAB’s decision erroneous, and appealed CRAB’s decision to the *453Superior Court.2 Before us is CRAB’s appeal from the allowance by a Superior Court judge of the board’s motion for judgment on the pleadings. We reverse the judgment of the Superior Court.

1. Facts. The facts are not in dispute. From October 12, 1988 to October 2, 1993, Barker worked as a revenue and finance planner for the Massachusetts Water Resources Authority (MWRA). As a member of the MWRA retirement system, she accrued four years and three months of creditable service with MWRA. When Barker resigned in 1993, she withdrew her “accumulated total deductions” from the MWRA retirement system. Subsequently, from March 15,1995 to June 30,1997, she worked as a contract employee for the Department of Environmental Protection (DEP). During that period, she was not a member of the SERS. In July, 1997, Barker entered full-time State service with the DEP and became a member of the SERS.

In March, 2001, pursuant to G. L. c. 32, § 3(8)(&), in relevant part set forth in the margin,3 Barker bought back her prior service with the MWRA by paying $19,008.60 to the board. *454This provision entitles her “to all creditable service resulting from [her] previous employment.”4 G. L. c. 32, § 3(8){b), as appearing in St. 1955, c. 695, § 5.

In October, 2006, Barker attempted a second buyback, this time pursuant to G. L. c. 32, § 4(l)(y), set forth in the margin,5 for her service from March 15, 1995 to June 30, 1997, when she worked as a contract employee of the DEP. That provision permits certain contract employees to “establish as creditable service up to 4 years [of their contract] service if the member [of the SERS] has 10 years of creditable service with the state employees’ retirement system” (emphasis supplied). G. L. c. 32, § 4(l)(s), inserted by St. 2006, c. 161, § 1. That Barker satisfied the other conditions of c. 32, § 4(l)(y), is not in dispute; the only issue is whether she met the ten-year requirement.

Barker retired on May 29, 2007. SERS allowed her credit for more than thirteen years of total service, including both her *455nine years of SERS service and her service in the MWRA retirement system, but not for service as a contract employee with the DER

2. Discussion. Our review of CRAB’s determination that Barker met the ten-year requirement is narrow and under a deferential standard. The retirement law is “notoriously complex,” Namay v. Contributory Retirement Appeal Bd.., 19 Mass. App. Ct. 456, 463 (1985), and we “will reverse only if [CRAB’s] decision was based on an erroneous interpretation of law or is unsupported by substantial evidence.” Foresta v. Contributory Retirement Appeal Bd., 453 Mass. at 676. Where an agency’s construction of a statute is reasonable, the court should not supplant it with its own interpretation. Ibid. Boston Retirement Bd. v. Contributory Retirement Appeal Bd., 441 Mass. 78, 82 (2004). See Damiano v. Contributory Retirement Appeal Bd., 72 Mass. App. Ct. 259, 261 (2008). See also Goldberg v. Board of Health of Granby, 444 Mass. 627, 633 (2005) (agency in Massachusetts has “considerable leeway in interpreting a statute it is charged with enforcing,” unless statute unambiguously bars the agency’s approach), quoting from Berrios v. Department of Pub. Welfare, 411 Mass. 587, 595-596 (1992).

We consider CRAB’s interpretation reasonable and may even be required. Creditable service is defined in G. L. c. 32, § 1, inserted by St. 1945, c. 658, § 1, as “all membership service, prior service and other service for which credit is allowable to any member under the provisions of sections one to twenty-eight inclusive.” Under G. L. c. 32, § 3(8)(b), Barker was entitled to her creditable service resulting from her employment by MWRA, and the board calculated her retirement benefits based on her more than thirteen years of service. See note 4, supra. CRAB’s decision that such thirteen years of creditable service to which Barker is “entitled” under G. L. c. 32, § 3(s)(6), satisfied the requirement of “10 years of creditable service with the state employees’ retirement system” is a reasonable reading of § 4(l)(s).

The board’s interpretation, adopted by the Superior Court, construes § 4(l)(s), to require that all creditable service be membership service with SERS. The words of the statute do not require such a limitation. Even if the words are ambiguous, CRAB’s interpretation must control.

*456We see nothing in § 4(l)(s) or in the record that bars CRAB’s interpretation. See Goldberg v. Board of Health of Granby, 444 Mass. at 633. Since CRAB’s construction of G. L. c. 32, § (4)(l)(y), is a reasonable one, Barker meets the ten-year requirement to buy back her contract services with the DEP under that provision. Accordingly, the judgment of the Superior Court is reversed, and a new judgment shall enter affirming CRAB’s decision.

So ordered.