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 cf478 {
    public  static  void main(String[] args)
    {
        int c1,c2,c3,c4,c5,r;
        Scanner sc=new Scanner(System.in);
        c1=sc.nextInt();
        c2=sc.nextInt();
        c3=sc.nextInt();
        c4=sc.nextInt();
        c5=sc.nextInt();
        int sum=c1+c2+c3+c4+c5;
        r=sum/5;
        if(sum%5==0)
        {
            if(r==0)
                System.out.print("-1\n");
            else
            System.out.print(r+"\n");
        }
        else
        {
            System.out.print("-1\n");
        }
    }
}
