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 {
  // main
  public static void main(String [] args) throws IOException {
    // makes the reader and writer
    BufferedReader f = new BufferedReader(new InputStreamReader(System.in));
    PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out));
    
    // read in 
    StringTokenizer st = new StringTokenizer(f.readLine());
    int total = 0;
    for (int i=0;i<5;i++) total+=Integer.parseInt(st.nextToken());
    
    // write to out
    if (total%5!=0 || total==0) out.println(-1);
    else out.println(total/5);
    
    // cleanup
    out.close();
    System.exit(0);
  }
}