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

    /* IMPORTANT: Multiple classes and nested static classes are supported */
     
    /*
     * uncomment this if you want to read input.
    //imports for BufferedReader
    import java.io.BufferedReader;
    import java.io.InputStreamReader;
     
    //import for Scanner and other utility classes
    import java.util.*;
    */
     
    // Warning: Printing unwanted or ill-formatted data to output will cause the test cases to fail
    import java.util.*;
    public class TestClass {
        public static void main(String args[] ) throws Exception {
            /* Sample code to perform I/O:
             * Use either of these methods for input
     
            //BufferedReader
            BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
            String name = br.readLine();                // Reading input from STDIN
            System.out.println("Hi, " + name + ".");    // Writing output to STDOUT
     
            //Scanner
            Scanner s = new Scanner(System.in);
            String name = s.nextLine();                 // Reading input from STDIN
            System.out.println("Hi, " + name + ".");    // Writing output to STDOUT
     
            */
     
            // Write your code here
            Scanner sc=new Scanner(System.in);
            int sum=0;
            for(int i=0;i<5;i++){
                sum+=sc.nextInt();
            }
            if(sum%5==0 && sum!=0){
                System.out.println(sum/5);
            }
            else
            System.out.println(-1);
        }
    }