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.*;
import java.io.IOException;

public class Main{
   public static void main(String[] args) {
    	try {

            java.io.BufferedReader r = new java.io.BufferedReader (new java.io.InputStreamReader (System.in));

            String[] inpStringArr = r.readLine().split(" ");
            int total =0;
            for(String i : inpStringArr){
                total+=Integer.parseInt(i);
            }
            int result = total/5;
	 
            if((total!=0) && (result*5 == total))
                System.out.println(result);
            else
                System.out.println("-1");

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}