#include<iostream>
#include<string>
#include<algorithm>
#include<cmath>
#include<math.h>
#include<iomanip>
#include<stdio.h>
#include<cstring>
#include<map>
using namespace std;
 
int main(){

  int px, py, qx, qy, dx, dy;
  cin >> px >> py >> qx >> qy;

  dx=abs(px-qx);
  dy=abs(py-qy);

  double ans=(dx+dy)/2.0;

  cout << fixed;
  cout << ans << endl;

  return 0;
}