#include<string>
#include<iostream>
#include<algorithm>
#include<set>
#include<math.h>
#include<vector>
#include<sstream>
#include<queue>
#include<functional>
#include<bitset>
#include<cstdio>
#include<iomanip>
using namespace std;

int main() {
	int px, py, qx, qy;
	cin >> px >> py >> qx >> qy;
	double x = (double)abs(px - qx) / 2;
	double y = (double)abs(py - qy) / 2;
	cout << setprecision(20) << x+y << endl;
	return 0;
}