= Sum Problem =
http://acm.hdu.edu.cn/showproblem.php?pid=1001

{{{#!cplusplus
/*written by czk*/
#include <stdio.h>

int main() {
    int n;
    while(scanf("%d", &n) != EOF)
        if(n%2)
            printf("%d\n\n", (n+1)/2*n);
        else
            printf("%d\n\n", n/2*(n+1));
}
}}}