import java.io.*;
import java.text.*;

class No410{
    public static void main(String[] args) {
        try{
            BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
            String[] str1 = br.readLine().split(" ");
            String[] str2 = br.readLine().split(" ");
            int[] A = new int[2];
            int[] B = new int[2];
            for(int i=0; i < 2; i++){
                A[i] = Integer.parseInt(str1[i]);
                B[i] = Integer.parseInt(str2[i]);
            }
            Calc(A,B); 
        }catch(IOException e){
            e.getStackTrace();
        }catch(NumberFormatException e){
            e.getStackTrace();
        }
    }
    
    public static void Calc(int[] a, int[] b){
        double ans = 0;
        ans = (double)(Math.abs(a[0]-b[0]) + Math.abs(a[1]-b[1])) / 2;
        DecimalFormat DF = new DecimalFormat("0.#");
        System.out.println(DF.format(ans));
    }
}