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.awt.image.SinglePixelPackedSampleModel;
import java.util.*;

public class practise {

    /*
     * Scanner scanner = new Scanner(System.in); int NCase = scanner.nextInt();
     * String Input = scanner.next();
     */
    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        int total = 0 ; 
        for (int i= 0 ; i < 5 ; i ++){
        
            total += scanner.nextInt() ; 
        
        }
        if (total%5!=0){System.out.println(-1);}
        else {System.out.println(total/5);}
       
        
        
    }
}

