import java.util.Scanner;

public class MineralWater2 {
    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 loadedBig = capacity/5;

        if(loadedBig>big){
            loadedBig=big;
        }


        int neededSmall = capacity-5*loadedBig;

        if(neededSmall > small){
            System.out.println("-1");
        }else{
            System.out.println(neededSmall);
        }
    }
}