#include <iostream>
#include <cstdlib>
using namespace std;

int main()
{
	double miles {0};
	cout << "How many miles do you need to travel today?\n";
	cin >> miles;

	if (!cin) {
		cerr << "Sorry, that wasn't a valid whole number.\n";
		return EXIT_FAILURE;
	}

	if (miles < 0) {
		cerr << "Sorry, you must travel some distance.\n";
		return EXIT_FAILURE;
	}

	double fare {3 + (miles - 1) * 3.5};

	cout << "That will be $" << fare << ".\n";

}