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;

public class Main {

	public static void main(String[] args) throws IOException {
		// TODO Auto-generated method stub

		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		String[] in1 = in.readLine().split(" ");
		int count = 0;
		for(int i = 0; i < in1.length; i++){
			count += Integer.parseInt(in1[i]);
		}
		if(count%5 == 0){
			System.out.print(count/5);
		}
		else{
			System.out.print(-1);
		}
	}

}
