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.StringTokenizer;

public class Solver {
    public void solve() {
        InputReader in = new InputReader(System.in);
        PrintWriter out = new PrintWriter(System.out);
        int c1 = in.nextInt();
        int c2 = in.nextInt();
        int c3 = in.nextInt();
        int c4 = in.nextInt();
        int c5 = in.nextInt();
        if ((c1 + c2 + c3 + c4 + c5) > 0 && (c1 + c2 + c3 + c4 + c5) % 5 == 0) {
            out.println((c1 + c2 + c3 + c4 + c5) / 3);
        }
        else {
            out.println(-1);
        }
        in.close();
        out.close();
    }

    public static void main(String... args) {
        new Solver().solve();
    }

    private static class InputReader {
        private BufferedReader reader;
        private StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }

        public void close() {
            try {
                reader.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}
