#include <iostream>
#include <cstdlib>
#include <cmath>
using namespace std;

void disp(unsigned long n, char *msg){
    cout << '\t' << n << msg;
}

unsigned long f(unsigned long n){
    if(!n)
	return 1;
    return n * f(n-1);
}

int main(int argc, char **argv){
    int num;
    if(argc>1)
	num = atoi(argv[1]);
    else{
	cout << "Enter a number to \nsee its factorial\n\t";
	cin >> num;
    }
    
    num>=0 ? disp(f(num)," is the factorial\n") : disp(num," must be greater than 0\n");

    return 0;
}