#include <iostream>
#include <cstdlib>
#include <cmath>
using namespace std;
typedef long long ll;
#define int ll

signed main(void) {
  double px, py, qx, qy, ans;

  cin >> px >> py;
  cin >> qx >> qy;

  cout << fabs((px - qx) + (py - qy)) / 2.0 << endl;
  return 0;
}