int doubleNumbers(int);

int main()
{
    int num;
    cout << "Give me a number and I'll DOUBLE IT.";
    cin >> num;
    //Call your function!
    doubleNumbers(num);

    return 0;
}

//Your function definition (What it does)
int doubleNumbers(int input);
{
    int double = input * 2;
    return double;
}