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

public class jjo
{

	public static void main(String args[])


	{   Scanner s=new Scanner(System.in);
		int a[]=new int[5];
		int sum=0;
		int ans=0;
		int flag=0;

		ArrayList<Integer>ar=new ArrayList<Integer>();
		for(int i=0;i<5;i++)
		{
			a[i]=s.nextInt();

			ar.add(a[i]);
			sum=sum+a[i];


		}

		for(int i=0;i<=100;i++)
		{


			if(i*5==sum)
			{  flag=1;
				ans=i;
				break;

			}
		}

		if((Collections.frequency(ar,ans)==5)|| (flag==0))
		{
			System.out.println("-1");
		}

		

		else
		{
			System.out.println(ans);
		}

	}
}