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.io.*;
public class A {

    /**
     * @param args
     * @throws IOException 
     */
    public static void main(String[] args) throws IOException {
        // TODO Auto-generated method stub
        BufferedReader k=new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer str=new StringTokenizer(k.readLine());
        
        int[]array = new int[5];
        int total = 0;
        for (int i = 0; i < array.length; i++) {
            array[i] = Integer.parseInt(str.nextToken());
            total += array[i];
        }
        if (total % 5 == 0 && total != 0) {
            System.out.println(total / 5);
        }
        else {
            System.out.println(-1);
        }

    }

}
