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

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */


import java.util.Scanner;


public class ProblemA {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args)
    {
     Scanner sc=new Scanner(System.in);
     int Sum=0;
     for(int i=0;i<5;i++)
     {
        Sum+=sc.nextInt();
     }
     
     if(Sum>=5 && Sum%5==0)
     {
         System.out.println(Sum/5);
     }
     else
     {
         System.out.println("-1");
     }
     
    }
}
