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;
import java.io.PrintStream;

/**
 * 
 * @author 
 *
 *
 *
 */

public class Main {
	public static void main(String[] args) throws IOException {
		MyScanner scanner = new MyScanner();
		PrintStream out = System.out;

		String string = scanner.next();
        String[] data = string.split(" ");

		int[] a  = new int[data.length];

		for (int i = 0; i < a.length; i++) {
			a[i] = Integer.parseInt(data[i]);
		}

		int sum = 0;
		for ( int i : a ) {
			sum += i;
		}

		int k = (sum % 5 == 0 ? sum / 5 : -1);

		out.println(k);

		scanner.reader.close();
		out.close();
	}

	// -----------MyScanner class for faster input----------
	public static class MyScanner {
		BufferedReader reader;

		public MyScanner() {
			this.reader = new BufferedReader(new InputStreamReader(System.in));
		}

		public void close() throws IOException {
			this.reader.close();
		}

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

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

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

		String next() {
			String str = "";
			try {
				str = this.reader.readLine().trim();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return str;
		}

		String[] nextStringArray() {
			String[] str = null;
			try {
				str = this.reader.readLine().trim().split(" ");
			} catch (IOException e) {
				e.printStackTrace();
			}
			return str;
		}

	}
}

