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;
public class Initial_Bet {

	public static void main(String[] args) {
		Scanner in=new Scanner (System.in);
		int c1=in.nextInt(),c2=in.nextInt(),c3=in.nextInt(),c4=in.nextInt(),c5=in.nextInt();		
		in.close();
		
		if((c1+c2+c3+c4+c5)%5!=0)
			System.out.println(-1);
		else
			System.out.println((c1+c2+c3+c4+c5)/5);
	}

}
