import java.util.Scanner;

public class Mock5_mineralWater {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        int smallBottles = scanner.nextInt();
        int bigBottles = scanner.nextInt();
        int truckCapacity = scanner.nextInt();

        while (bigBottles > 0 && truckCapacity >= 0) {
            if (truckCapacity < 5) {
                break;
            }
            bigBottles--;
            truckCapacity -= 5;
        }


        int difference = truckCapacity - smallBottles;

        if (difference > 0) {
            System.out.println("-1");
        } else if (difference == 0) {
            System.out.println(smallBottles);
        } else {
            System.out.println(truckCapacity);
        }
    }
}