Exercise 1 Done (Again!)

#include <iostream>
using namespace std;
const int SENTINEL = -999;

int main()
{
    double n = 0, sum = 0, temp = 1;

    cout << "Enter a number(Enter -999 to exit): ";
    cin >> n;
    cout << endl;

    while (n != SENTINEL)
    {
        if(n < 0)
        {
            cout << "Only positive numbers!" << endl;
        }
        else if(!cin.good())
        {
            cout << "No letters!" << endl;
            cin.clear();
            cin.ignore(100, '\n');
        }
        else if (n == 0)
        {
            sum = 1;
            cout << sum << endl;
        }
        else
        {
            while (n != 0)
            {
                temp = temp * n;
                n = n - 1;
            }
            cout << temp << endl;
        }

        temp = 1;
        cout << "Enter a number(Enter -999 to exit): ";
        cin >> n;
        cout << endl;
    }

    return 0;
}