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

int main()
{
	int gallons {0};
	cout << "How many gallons do you need?\n";
	cin >> gallons;

	if (!cin) {
		cerr << "Sorry, that wasn't a valid number.\n";
		return EXIT_FAILURE;   //gallons dies here; price not born yet.
	}

	if (gallons < 1) {
		cerr << "Why are you here? You don't need gas.\n";
		return EXIT_FAILURE;   //gallons dies here; price not born yet.
	}

	int price {  5 * gallons };

	cout << "The price of your fillup will be "
		<< price << " USD.\n";

	return EXIT_SUCCESS;   //gallons and price die here.
}