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.util.*;
import java.io.*;

public class Bet {
    public static void main(String args[]) {
    	Scanner cin = new Scanner(System.in);
	
    	int c1 = cin.nextInt();
    	int c2 = cin.nextInt();
    	int c3 = cin.nextInt();
    	int c4 = cin.nextInt();
    	int c5 = cin.nextInt();

int ct = c1+c2+c3+c4+c5;

if ( ct % 5 == 0 ) {
	System.out.println(ct/5);
} else {
	System.out.print(-1);
}
    	
    	cin.close();
    }
}