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.*;


public class Test{
    

    public static void main(String args[]) {
     Scanner in = new Scanner(System.in); 
     int c1=in.nextInt(),c2=in.nextInt(),c3=in.nextInt(),c4=in.nextInt(),c5=in.nextInt();
    
     if(c1==c2 && c2==c3 && c3==c4 && c4==c5 && c1==0)
            System.out.println("-1");
     else   if((c1+c2+c3+c4+c5)%5 == 0)
            System.out.println((c1+c2+c3+c4+c5)/5);
     else
            System.out.println("-1");
   
   
   
   
   
   
   
   
   
    }
}
