99 100 100 100 100
 57 83 11 4 93
 99 99 99 99 99
 100 0 0 0 0
 0 1 2 3 4
 93 100 99 90 98
 87 38 19 33 100
 1 1 1 1 1
 0 0 0 0 1
 2 3 4 5 6
 1 2 1 2 3
 0 0 0 0 0
 100 100 100 100 100
 43 83 1 0 23
 43 83 1 100 23
 56 0 0 0 4
 99 98 98 99 100

-1
 -1
 99
 20
 2
 96
 -1
 1
 -1
 4
 -1
 -1
 100
 30
 50
 12
 -1

import java.util.Scanner;
public class Main {
    public static void giveAnAnswer(String wtr){
        System.out.println(wtr);
        System.exit(0);
    }
    public static void main(String[] args) {
        // write your code here
        Scanner in = new Scanner(System.in);
        int c1= in.nextInt();
        int c2= in.nextInt();
        int c3= in.nextInt();
        int c4= in.nextInt();
        int c5= in.nextInt();
        int summ = c1 + c2 + c3 + c4 + c5;
        if(summ % 5 != 0){
            giveAnAnswer("-1");
        }else{
            giveAnAnswer((summ / 5) + "");
        }

    }

}
