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.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

/**
 * Created by shubheksha on 23/3/15.
 */
public class P478A {
    public static void main(String[] args) throws IOException {
        BufferedReader o = new BufferedReader(new InputStreamReader(System.in));
        String[] in = o.readLine().split(" ");
        int i, sum = 0;
        for (i = 0; i < 5; i++) {
            sum += Integer.parseInt(in[i]);
        }
        if (sum % 5 != 0 || sum == 0)
            System.out.print("-1");
        else
            System.out.print(sum/5);
    }
}
