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.lang.*;
 
public class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
		Scanner sc = new Scanner(System.in);double sum=0;byte c;byte i;
		for( i=0;i<5;++i){
			c=sc.nextByte();
			if(c>=0)
				sum+=c;
			else
				break;
		}
           double n = sum/5;
           if(sum<=0 || i!=5)
        	   System.out.println(-1);
           else
        	   System.out.println(n==(int)n?(int)n:-1);
	}
}