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) {
     int a1,a2,a3,a4,a5,k;
     float tot,m;


     Scanner sc=new Scanner(System.in);
     a1=sc.nextInt();
     a2=sc.nextInt();
     a3=sc.nextInt();
     a4=sc.nextInt();
     a5=sc.nextInt();

         if((0<=a1&&a1<=100)&&(0<=a2&&a2<=100)&&(0<=a3&&a3<=100)&&(0<=a4&&a4<=100))
         {
     tot=a1+a2+a3+a4+a5;
    
     m=tot/5;
  
     k=(int)m;
     
     if(k==m)
         System.out.println(+k);
     else
         System.out.println(-1);


    }
    }

}