/* Bai tap 1_20 - Tinh n!! */

#include <stdio.h>

void main()

{

  int n, start, i;

  unsigned long gthua = 1;

  printf("

Nhap gia tri N : ");

  scanf("%d", &n);

  if (n%2 == 0)

    start = 2;

  else

    start = 1;

  for (i=start; i<=n; i = i+2)

    gthua *= i;

  printf("

%d!! = %ld", n, gthua);

  getch();

}