My idea is this: the first stroke 1+2+...+n, which is the knowledge of arithmetic series in senior high school, but if you have studied olympiad in primary school, it should not be a problem. The result is n(n+ 1)/2, n and n+ 1 coprime, so n and n+60/. Under the condition that n is a positive odd number, there is always an n+b n = (a+b) (a (n-1)-a (n-2) b+a (n-3) b 2+…-ab (n-2)+B. In addition, if/kloc-.