C++ help

Just started C++, Im required to make a program that finds the factorial of a entered number.

I keep getting this error
"`main' must return `int'"

Google wasn't much use

Heres my code:

#include <iostream>
using namespace std;

int factorial( int num);
void main()
{
    cout <<"Enter a number to find the factorial of"<<endl;
        int num, ans;
        cin >>num;
        ans = factorial(num);
        cout <<"The value of the factortial of " <<num <<" is " <<ans<<endl;
}


int factorial( int num)
{
    int fact = 1;
        for( int i = fact + 1; i <=(num + 1); i++)
        {
        fact = fact * i;
        }
    return fact;
}

>>6
http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-11.html#%_fig_1.4