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.*;
import java.lang.*;
import java.io.*;
public class Main
{
    public static int mod=(int)Math.pow(10,9) + 7;
    // public static ArrayList<Integer> a=new ArrayList<>();
	public static void main (String[] args) throws java.lang.Exception
	{
		try{
            BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
            PrintWriter ot=new PrintWriter(System.out);
            int t=1;//Integer.parseInt(br.readLine().trim());
            while(t-->0)
            {
                String s[]=br.readLine().trim().split(" ");
                int sum=0;
                // double sum=0;
                for(int i=0;i<5;i++)
                sum+=Integer.parseInt(s[i]);
                if(sum%5==0)
                ot.println(sum/5);                
                else
                ot.println(-1);
                
            }
        
            ot.close();
            br.close();
            
        } catch(Exception e){
            e.printStackTrace();
            return;
        }
    }
    

}

    