#include <algorithm>
#include <cstdio>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
using namespace std;

#define ll long long
#define INF (1 << 30)
#define INFLL (1LL << 60)

int main() {
	double x1,x2,y1,y2;
	cin >> x1 >> y1 >> x2 >> y2;

	x1 *= -1.0;

	double ans = y1 - ((y2 - y1) / (x2 - x1) * x1);
	double ans2 = y2 - ((y2 - y1) / (x2 - x1) * x2);

	cout << (ans + ans2) / 2.0 << endl;
	return 0;
}