S.D. Codified Laws § 3-12C-1112
Any foundation member who retires before being eligible for social security retirement benefits may elect to receive initial retirement benefit payments from the system in an amount greater than the standard benefit payments computed on the basis of the member's age and earnings at retirement. The greater amount, in conjunction with a later reduced amount, shall be the actuarial equivalent of the normal retirement benefit computed on the basis of age at retirement. The greater amount shall be paid until the foundation member reaches the age of sixty-two, at which time the payment from the system shall be the reduced amount so that, as far as possible, the foundation member's combined monthly retirement income from the system and social security shall approximately equal the greater amount paid prior to age sixty-two.
SDCL 3-12C-1112