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.PrintWriter;
import java.util.*;

public class contest3 {
	public static void main (String [] args) throws IOException{
		BufferedReader br = new BufferedReader( new InputStreamReader(System.in));
		PrintWriter out = new PrintWriter(System.out);
		StringTokenizer st = new StringTokenizer(br.readLine());
		int [] coins = new int [5];
		int sum  = 0;
		for (int i = 0 ; i<5 ; i++){
			coins[i] = Integer.parseInt(st.nextToken());
			sum+=coins[i];
		}
		out.println((sum%5==0)?sum/5:-1);
		out.flush();
		out.close();
	}
}
