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

class Process {
    private int[] P;
    private int[] Q;

    Process(int[] P, int[] Q) {
        this.P = P;
        this.Q = Q;
    }

    double getResult() {
        int distance = Math.abs(P[0] - Q[0]) + Math.abs(P[1] - Q[1]);

        return (distance / 2.0);
    }
}

public class Main {
    public static void main(String[] args) throws IOException {
        var bufferedReader = new BufferedReader(new InputStreamReader(System.in));
        var printWriter    = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));

        // 入力
        int[] P = Stream.of(bufferedReader.readLine().trim().split("[ ]+")).mapToInt(Integer::parseInt).toArray();

        int[] Q = Stream.of(bufferedReader.readLine().trim().split("[ ]+")).mapToInt(Integer::parseInt).toArray();

        // 処理および出力
        printWriter.println((new Process(P, Q)).getResult());
        
        bufferedReader.close();
        printWriter.close();
    }
}