Factorial of given prime number

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

int main()
{
    int num;
    bool prime = true;
    cin>>num;
    for(int i=2; i<=(num/2); i++)
        if(num%i==0)
        {
            prime = false;
            break;
        }
    if(prime==true)
    {
        int a=1;
        for(int i=1; i<=num; i++){
            a*=i;
        }
        cout<<a;
    }
    else cout<<"Error! Not a prime number.";
    return 0;
}