//Normal

#define _GLIBCXX_DEBUG
#define ll long long
#include <bits/stdc++.h>
using namespace std;
using Graph = vector<vector<int>>;

void print(auto a){
  cout << a;
}

void prints(auto a){
  cout << a << " ";
}

void prints(){
  cout << " ";
}

void printl(auto a){
  cout << a << endl;
}

void printl(){
  cout << endl;
}

void fix(int n){
  cout << fixed << setprecision(n);
}

int main(){
  int PX, PY, QX, QY; cin >> PX >> PY >> QX >> QY;
  
  int sum = abs(PX-QX) + abs(PY-QY);
  
  double ans = (double)sum/2.0;
  
  fix(5);
  printl(ans);
  
  return 0;
}