import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.stream.Stream;

public class No00000410_Main {
	static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	public static void main(String[] args) throws IOException {
		int[] p = Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();
		int[] q = Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();
		System.out.println((Math.abs(p[0] - q[0]) + Math.abs(p[1] - q[1])) / 2.0);
	}
}