import java.io.*; import java.util.*; public class Main { public static void main(String[] args) throws Exception { Scanner sc = new Scanner(); int x1 = sc.nextInt(); int y1 = sc.nextInt(); int x2 = sc.nextInt(); int y2 = sc.nextInt(); double left = Math.min(y1, y2); double right = Math.max(y1, y2); for (int i = 0; i < 10000000; i++) { double m1 = (left * 2 + right) / 3; double m2 = (left + right * 2) / 3; if (Math.sqrt(Math.pow(x1, 2) + Math.pow(y1 - m1, 2)) + Math.sqrt(Math.pow(x2, 2) + Math.pow(y2 - m1, 2)) < Math.sqrt(Math.pow(x1, 2) + Math.pow(y1 - m2, 2)) + Math.sqrt(Math.pow(x2, 2) + Math.pow(y2 - m2, 2))) { right = m2; } else { left = m1; } } System.out.println(left); } } class Scanner { BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); StringTokenizer st = new StringTokenizer(""); StringBuilder sb = new StringBuilder(); 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(); } }