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.io.*;
import java.util.*;
public class ppp1
{
    public static void main(String[] args) throws IOException
    {
    	Scanner f = new Scanner(System.in);
        StringTokenizer st = new StringTokenizer(f.nextLine());
        int n = 0;
        for(int i = 0; i<5;i++){
        	n += Integer.parseInt(st.nextToken());
        }
        //System.out.println(n);
        
        f.close();
        
        if(n == 0 || n%5 != 0){
        	System.out.println(-1);
        }
        else{
        	System.out.println(n/5);
        }
        
    }
}