#include <stdio.h>
#include <stdlib.h>

int f(int);

int main(int argc, char *argv[])
{
  printf("%d\n", f(atoi(argv[1])));
  return 0;
}

int f(int N)
  {
    if (N == 0) return 0;
    return f(N-1) + N;
  }