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OEIS

A. Alex and a Rhombus

Quiz https://codeforces.com/contest/1180/problem/A AC Code https://codeforces.com/contest/1180/submission/55876143 解法 数列は1, 5, 13, 25, ... OEISに聞く https://oeis.org/search?q=1%2C+5%2C+13%2C+25&language=english&go=Search 計算量 O(1)