#include <bits/stdc++.h>

using namespace std;

struct Initializer {
  Initializer() {
    cin.tie(0);
    ios::sync_with_stdio(0);
    cout << fixed << setprecision(15);
  }
} initializer;

int main() {
  int px, py, qx, qy;
  cin >> px >> py >> qx >> qy;
  cout << (abs(px - qx) + abs(py - qy)) / 2.0 << endl;
}