((n-1)n/2, (n+1)(n+2))
=(n(n-1)/2, 2n+1)　[右-左]
=(4n(n-1), 2n+1)　[右は奇数：左x2^3]
=((2n+1)(2n-3)+3, 2n+1)　[nの多項式と思って左÷右]
=(3,2n+1)　[左-右x(2n-3)]
=3 or 1