import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int xa = sc.nextInt();
        int ya = sc.nextInt();
        int xb = sc.nextInt();
        int yb = sc.nextInt();
        double left = 0;
        double right = 1000;
        for (int i = 0; i < 1000000; i++) {
            double m1 = (left * 2 + right) / 3;
            double m2 = (left + right * 2) / 3;
            double d1 = Math.sqrt(Math.pow(xa, 2) + Math.pow(ya - m1, 2)) + Math.sqrt(Math.pow(xb, 2) + Math.pow(yb - m1, 2));
            double d2 = Math.sqrt(Math.pow(xa, 2) + Math.pow(ya - m2, 2)) + Math.sqrt(Math.pow(xb, 2) + Math.pow(yb - m2, 2));
            if (d1 < d2) {
                right = m2;
            } else {
                left = m1;
            }
        }
        System.out.println(left);
    }
}
class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public double nextDouble() throws Exception {
        return Double.parseDouble(next());
    }
    
    public String next() throws Exception {
        while (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
}