#include <bits/stdc++.h>

using namespace std;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int px, py, qx, qy;
  cin >> px >> py >> qx >> qy;
  int sum = abs(px - qx) + abs(py - qy);
  cout << fixed << setprecision(20) << sum / 2 + (sum % 2 == 0 ? 0 : 0.5)
       << '\n';
  return 0;
}