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.StringTokenizer;
public class CF0478A
{
 public static void main(String[] helloWorld) throws IOException
 {
  BufferedReader sIn = new BufferedReader(new InputStreamReader(System.in));
  StringTokenizer sT = new StringTokenizer(sIn.readLine());
  int sum = 0;
  for (int i = 0; i < 5; i ++)
   sum += Integer.parseInt(sT.nextToken());
  System.out.println((sum % 5 == 0) ? sum / 5 : "-1");
 }
}
