#include <stdio.h>
#include <math.h>

int factorial(int x);

main()
{

  int x, fact_x;
  double a;

  printf("enter x\n");
  if ( scanf("%d", &x) != 1 || x < 0)
    {
      printf("error - you must enter a positive integer\n");
      exit(1);
    }

  fact_x = factorial(x);
  printf("factorial %d is %d\n", x, fact_x);

}

int factorial(int x)
{
  int i, factx = 1;

  for(i=1; i<=x ; i++ )
    factx *= i;
  return factx;
}