goal (from e.c.m.): evaluate 1^2+2^2+3^2-4^2-5^2+6^2+7^2+8^2-9^2-10^2+...-2010^2, where each three consecutive + must be followed by two - (^ meaning ** in this context) my solution: >>> s = 0 >>> for i in range(1, 2011): ... s += i**2 ... if not (i+1)%5: ... s -= 2*i**2 ... if not i%5: ... s -= 2*i**2 ... >>> print s 536926141 >>> bye