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 void soln(int c1, int c2, int c3, int c4, int c5){
            int sum = c1+c2+c3+c4+c5;
            if(sum%5 == 0){
                System.out.println(sum/5);
            }
            else{
                System.out.println(-1);
            }

    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int c1,c2,c3,c4,c5;

        c1 = sc.nextInt();
        c2 = sc.nextInt();
        c3 = sc.nextInt();
        c4 = sc.nextInt();
        c5 = sc.nextInt();
        Main obj = new Main();
        obj.soln(c1,c2,c3,c4,c5);

    }


}
