#include <bits/stdc++.h>
using namespace std;
int main(){
  cout << fixed << setprecision(5) << endl;
  int PX, PY, QX, QY;
  cin >> PX >> PY >> QX >> QY;
  cout << (double) (abs(QX - PX) + abs(QY - PY)) / 2 << endl;
}