#include <iostream>

using namespace std;

int main()
{
    double bill;
    double tip;

    // Keep processing bills, until the user enters a 0.
    do {
        cout << "Enter the bill amount (0 to exit):";
        cin >> bill;

        // Quit the program if the person enters a negative number.
        if(bill<0){
            cout << "Invalid number!" << endl;
            exit(-1);
        }

        // Valid bill has been entered.
        if(bill>0) {
            tip=bill*.15;

            cout << "Tip is: $" << tip << endl;
        }

    }while(bill!=0);

    return 0;
}