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());
        int big = Integer.parseInt(scanner.nextLine());
        int capacity = Integer.parseInt(scanner.nextLine());
        int result = -1;


        big = big * 5;

        if (capacity == big) {
            result = 0;
            System.out.println(result);
        }
        else if( capacity > big + small){
            System.out.println(result);
        }
        else {
            result = small;
            System.out.println(result);
        }

    }

}