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


public class Main_yukicoder410 {

    public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	Printer pr = new Printer(System.out);

    	int px = sc.nextInt();
    	int py = sc.nextInt();
    	int qx = sc.nextInt();
    	int qy = sc.nextInt();

    	double tmp = (double)(Math.abs(px - qx)) / 2 + (double)(Math.abs(py - qy)) / 2;

    	pr.println(tmp);

        pr.close();
        sc.close();
    }

	@SuppressWarnings("unused")
	private static class Scanner {
		BufferedReader br;
		Iterator<String> it;

		Scanner (InputStream in) {
			br = new BufferedReader(new InputStreamReader(in));
		}

		String next() throws RuntimeException  {
			try {
				if (it == null || !it.hasNext()) {
//					it = Arrays.asList(br.readLine().split(" ")).iterator();
					it = Arrays.asList(br.readLine().split("\\p{javaWhitespace}+")).iterator();
				}
				return it.next();
			} catch (IOException e) {
				throw new IllegalStateException();
			}
		}

		int nextInt() throws RuntimeException {
			return Integer.parseInt(next());
		}

		long nextLong() throws RuntimeException {
			return Long.parseLong(next());
		}

		float nextFloat() throws RuntimeException {
			return Float.parseFloat(next());
		}

		double nextDouble() throws RuntimeException {
			return Double.parseDouble(next());
		}

		void close() {
			try {
				br.close();
			} catch (IOException e) {
//				throw new IllegalStateException();
			}
		}
	}

	private static class Printer extends PrintWriter {
		Printer(PrintStream out) {
			super(out);
		}
	}
}