#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;

int main() {
  int px, py, qx, qy;
  cin >> px >> py >> qx >> qy;
  int d = abs(px - qx) + abs(py - qy);
  cout << fixed << setprecision(1) << d / 2.0 << endl;
}