#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;

int main() {
  double PX, PY, QX, QY; cin >> PX >> PY >> QX >> QY;
  int X = abs(PX-QX);
  int Y = abs(PY-QY);
  double ans = (X+Y)/2.0;
  cout << ans << endl;
}