牛牛数列4

其实就是等差数列求和而已
n有多大，就求多少次和
#include <stdio.h>

int main()
{

int n;
int sum = 0;
int i = 1;
int sign = 1;
scanf("%d",&n);
for(i=1;i<=n;i++)
{
    sum += i*(1+i)/2;

}
printf("%d\n",sum);

return 0;

}