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

/* package whatever; // don't place package name! */

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

/* Name of the class has to be "Main" only if the class is public. */
public class InitialBet478A
{
	public static void main (String[] args) throws java.lang.Exception
	{
		int x1, x2, x3, x4, x5, total;
		Scanner input=new Scanner(System.in);
		x1=input.nextInt();
		x2=input.nextInt();
		x3=input.nextInt();
		x4=input.nextInt();
		x5=input.nextInt();
		total= x1 + x2 + x3 + x4 + x5;
		if(x1==0 && x2==0 && x3==0 && x4==0 && x5==0)
		System.out.println("-1");
		else if(total %5 ==0)
		System.out.println(total/5);
		else
		System.out.println("-1");
	}
}