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.*;

public class GFG {
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		Scanner sc=new Scanner(System.in);
		int sum=0,n;
	//	if(shr.charAt(0)>mor.charAt(0))
		//	System.out.println("f yeah");
		for(int i=0;i<5;i++)
		{
			n=sc.nextInt();
			sum+=n;
		}
		if(sum%5!=0 || sum==0)
		System.out.println("-1");
		else
			System.out.println(sum/5);
	}
}
