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.io.PrintWriter;
import java.util.Scanner;

public class A {

    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int a1 = in.nextInt();
        int a2 = in.nextInt();
        int a3 = in.nextInt();
        int a4 = in.nextInt();
        int a5 = in.nextInt();

        int s = a1 + a2 + a3 + a4 + a5;

        if (s == 0){
            out.print(0);
        } else if(s % 5 == 0){
            out.print(s/5);
        } else {
            out.print(-1);
        }

        out.flush();
    }
}
