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 Main
{ 

	public static void main(String[] args)
	throws Exception
	{ 

		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		PrintWriter writer = new PrintWriter(System.out);
		StringTokenizer sz = new StringTokenizer(reader.readLine());

		int sum=0;

		for (int i=1;i <= 5;++i)
			sum += Integer.parseInt(sz.nextToken());

		if (sum % 5 == 0 && sum != 0)
			writer.print(sum / 5);
		else
			writer.print(-1);

		writer.flush();
	} 

}
