4

#include <stdio.h>
int sum(int n)
{
    int i, sum = 0;
    for(i = 1; i <= n; i++)
    {
        sum += i;
    }
    return sum;
}
int main()
{
   int n;
   scanf("%d", &n);
   printf("sum = %d\n", sum(n));
    return 0;
}