wrong result calculation

Question

hi, when i run this code which is taking a number and calculating it's factorial, when i try and enter a number more than 16 it gives me a false negative number and sometimes zero.
why is this and how to fix? but i just wanna know why it does this really.
here's the code:

#include <iostream>
using namespace std;
int fact(int n){
if (n==1){
return 1;
}
else {
return n*fact(n-1);
}
}
int main(){
int a;
cin >>a;
cout << fact(a);
}