#include <bits/stdc++.h>
#define rep(i, ss, ee) for (int i = ss; i < ee; ++i)
using namespace std;

double solve() {
  int px, py, qx, qy;
  cin >> px >> py >> qx >> qy;
  double ans = (abs(px - qx) + abs(py - qy)) / 2.0;
  return ans;
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << solve() << endl;
  getchar();
}