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

/**
 * 
 * 1   1   1
 * 2 5 4 0 4
 * 3
 * 
 * 
 * 
 * */
public class AInitialBet {
    public static void main(String[]arg) throws IOException
    {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st = new StringTokenizer(in.readLine());
        int[]arr = new int[5];
        int total  = 0;
        for(int i = 0; i < 5; i++)
        {
            arr[i] = Integer.parseInt(st.nextToken());
            total += arr[i];
        }
        if(total%5 ==0 && total != 0)
            System.out.println(total/5);
        else
            System.out.println(-1);
    }
}
