函数题：Sum 1-n

Write a function to calculate the sum of 1 - n.
### function declaration：
c++
int sum ( int n );


### Test code：
c++
#include <stdio.h>
int sum (int n);
int main()
{
int n;
scanf("%d", &n);
printf("%d", sum(n));
return 0;
}

/* Your answer will be put here */


### Input sample：

5


### Output sample：

15






答案：若无答案欢迎评论