#include <stdio.h>
#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
#include <functional>
#include <map>
#include <iomanip>
#include <math.h> 
#include <stack>
#include <queue>
#include <bitset>
#include <cstdlib>
#include <tuple>
#include <cctype>
#include <ctype.h>
#include <set>
#include <sstream>

using namespace std;


int main() {
	int i, j, k;
	double px, py, qx, qy;

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

	double timex = abs(px - qx) / 2;
	double timey = abs(py - qy) / 2;

	cout << timex + timey << endl;




	getchar();
	getchar();
	return 0;
}