v1.1

import java.util.Scanner;

public class MineralWater {

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        int small = Integer.parseInt(scanner.nextLine()); //4
        int big = Integer.parseInt(scanner.nextLine()); //0
        int capacity = Integer.parseInt(scanner.nextLine()); //11

        int result = capacity - (big * 5 + small); // 11 - (10 + 4) = -3

        if (result <= 0) {
            if( result == 0) {
                System.out.print(small); // all loaded
            } else if (capacity < (big *5) || capacity < small ){
                System.out.println(-1);
            }
            else {
                System.out.println(small + result); //small (4) + result (-3) = 1
            }
        } else {
            System.out.println(-1); // not enough bottles for the load
        }

    }
}