View Code of Problem 30

#include <stdio.h>

long long int sum(long long int n)
{
    int sum;
    if(n % 2 == 0)
        sum = n / 2 * (n + 1);
    else
        sum = (n + 1) / 2 * n;
    return sum;
}

int main(void)
{
  long long int n;
  while(scanf("%d",&n) != EOF)
  {
    printf("%lld\n",sum(n));
  }
  return 0;
}