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.Scanner;

/**
 *
 * @author Mostafa
 */
public class Test62 {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        Scanner in = new Scanner(System.in);
        int x = in.nextInt() ;
        int x1 = in.nextInt() ;
        int x2 = in.nextInt() ;
        int x3 = in.nextInt() ;
        int x4 = in.nextInt() ;
        if ( ( x + x1 + x2 + x3 + x4 ) % 5 == 0 && (( x + x1 + x2 + x3 + x4 ) != 0) )
            System.out.println((x + x1 + x2 + x3 + x4) / 5);
        else
            System.out.println(-1);
    }
    
}