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 SolveContest {
    private BufferedReader in;
    private PrintWriter out;

    SolveContest() {
        try {
            in = new BufferedReader(new FileReader("input.txt"));
            out = new PrintWriter(new FileWriter("output.txt"),true);
            solve();
        } catch (Exception e) {
            e.printStackTrace();
            in = new BufferedReader(new InputStreamReader(System.in));
            out = new PrintWriter(System.out);
            solve();
        } finally {
            try {
                in.close();
            } catch (IOException e) {

            }
            out.close();
        }
    }
    StringTokenizer st;
    String nextToken() {
        while (st == null || !st.hasMoreTokens())
            try {
                st = new StringTokenizer(in.readLine());
            } catch (IOException e) {

            }
        return st.nextToken();
    }

    int nextInt() {
        return Integer.valueOf(nextToken());
    }
    long nextLong() {
        return Long.valueOf(nextToken());
    }

    public void solve() {
        int sum = 0;
        for (int i = 0; i < 5; i++) {
            sum += nextInt();
        }
        out.print(sum % 5 == 0?sum/5:-1);
    }


    public static void main(String[] args) {
        new SolveContest();
    }
}