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.BufferedOutputStream;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;


public class Initial {
public static void main(String[]args) throws IOException{
    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 c1=Integer.parseInt(st.nextToken());
        int c2 = Integer.parseInt(st.nextToken());
        int c3= Integer.parseInt(st.nextToken());
        int c4= Integer.parseInt(st.nextToken());
        int c5 = Integer.parseInt(st.nextToken());
        int sum=c1+c2+c3+c4+c5;
        if(sum%5==0||sum==0){
            out.print(sum/5);
        }else{
            
            
        out.println(-1);
        }
        out.close();
        f.close();
    
}
    
    
    
}
