Fastest possible algorithm to sum numbers up to N [closed]

Question

I want a really fast algorithm or code in C to do the following task: sum all numbers from 1 to N for any given integer N, without assuming N is positive. I made a loop summing from 1 to N, but it is too slow.

Answer 1

If N is positive: int sum = N*(N+1)/2;

If N is negative: int tempN = -N; int sum = 1 + tempN*(tempN+1)/2 * (-1);.

Answer 2

sum = N * (N + 1) / 2