#include<stdio.h>
    fac(int n) reentrant {
      if (n<1)  return(1);
      else      return(n*fac(n-1));
     }
  
    main() {
      int n;
      printf("please input a number: \n");
      scanf("%d", &n);
      printf("fac(%d)=%d\n", n, fac(n));
      while(1);
     }