IMO 1986 LL CZS16

Given a positive integer k, find the least integer nk for which

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IMO 1986 LL CZS16

Origin: CZS

Problem

Given a positive integer k, find the least integer nk for which there exist five sets S1, S2, S3, S4, S5 with the following properties: |Sj| = k for j = 1, . . . , 5, j=1 Sj = nk; |Si \capSi+1| = 0 = |S5 \capS1|, for i = 1, . . . , 4.