C :

#include<stdio.h>   

int main(){ 
	int n,i,sum=1;
	scanf("%d",&n);
	
	
	for(i = 1;i <= n;i++){ 
		sum *= i;  //等同于 sum = sum * i； 
	}
	
	printf("%d",sum);
	return 0;
} 

C++ :

#include <iostream>
using namespace std;

int main(){
	int i,s = 1,n;
	cin>>n;
	for(i = 1;i <= n;i++){
		s = s * i;
	}
	
	cout<<s<<endl;
	return 0;
}

Python :

n = int(input())
i = 1;
s = 1
while i <= n:
    s = s * i
    i += 1;
print(s)