1-2+3-4.+99-100

i <=100
i*j
j = -j

#include <stdio.h>
int main()
{
    int i = 1, j = 1, s = 0;
    while(i <= 100)
    {
        s += i*j;
        i++;
        j = -j;
    }
    printf("s = %d\n",s);
}