java做很轻松。。。
import java.util.Scanner;
import java.math.BigInteger;
public class Main
{
public static void main(String[]args){
Scanner scanner = new Scanner(System.in);
BigInteger ans;
int n;
while(scanner.hasNextInt()) {
n = scanner.nextInt();
ans = BigInteger.ONE;
for(int i = 2; i <= n; i++) ans = ans.multiply(BigInteger.valueOf(i));
System.out.println(ans);
}
}
}