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.Scanner;
public class Main
{
    public static void main(String args[])throws Exception
    {
       Scanner sc=new Scanner(System.in);
       int a=sc.nextInt();
       int b=sc.nextInt();
       int c=sc.nextInt();
       int d=sc.nextInt();
       int e=sc.nextInt();
       int t=(int)Math.ceil(((a+b+c+d+e)/(5.0)));
       int x=(a+b+c+d+e)/5;
       if(t==x && t!=0)
           System.out.print(t);
       else
           System.out.print(-1);
    }
}