#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

int main() {
  double px, py, qx, qy;
  cin >> px >> py >> qx >> qy;
  double xdiff = abs(px - qx);
  double ydiff = abs(py - qy);  
  double ans = xdiff / 2;
  ans += ydiff / 2;
  cout << fixed << setprecision(10) << ans << '\n';
  return 0;
}