#include<bits/stdc++.h>
using namespace std;
int main(){
    long long n;
    cin>>n;
    long long y=1,u=1;
    for(int i=1;i<=n;i++){
        u=u*y;
        y++;
    }
    cout<<u;
    return 0;
}

C++ :

#include <cstdio>
using namespace std;
int main ()
{
	long long s;		//long long的范围为-263～263-1，
					    //int的-1019～1019略窄
	int n;				//n不能定义为long long，否则for语句死循环
	s=1;
	scanf("%d",&n);
	for (int i=1; i<=n ; ++i)	//当n=13时s超出long int的范围
		s*=i;
	printf("%lld\n",s);		//使用%d输出long long结果有误
	return 0;
}

Python :

# coding=utf-8
s = int(input())
for i in range(1,s,1):
    s=s*i
print(s)