https://ideone.com/SCiVpm

#include<bits/stdc++.h>
using namespace std;

int fact(int n){
    ///cout<<n<<endl;
    
    if(n < 0) return -1;
    if(n==0) return 1;

     int smallAns = fact(n-1);
     return n*smallAns;
}

int main(){
     int n;
     cin>>n;

     int ans = fact(n);
     cout<<ans<<endl;
    return 0;
}