#include <iostream>
#include <stdio.h>
#include <algorithm>
#include <vector>
#include <cmath>
#include <string>
#include <sstream>
#include<sstream>
#include <queue>
#include <limits>
#include <cstdlib>
#include <deque>
#include <map>
#include <set>
using namespace std;

int main() {


	double PX =0;
	double PY =0;
	double QX =0;
	double QY =0;
	cin>>PX>>PY>>QX>>QY;

	double ans =((abs(PX-QX))+(abs(PY-QY)))/2;

	cout<<ans<<endl;

	return 0;
}