#define _CRT_SECURE_NO_WARNINGS
#include<sstream>
#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<climits>
#include<cmath>
#include<string>
#include<vector>
#include<set>
#include<map>
#include<queue>
#include<numeric>
#include<functional>
#include<algorithm>
#include<bitset>
#include<tuple>
#include<unordered_set>
#include<random>
using namespace std;
#define INF (1<<29)
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define all(v) v.begin(),v.end()
#define uniq(v) v.erase(unique(all(v)),v.end())
#define indexOf(v,x) (find(all(v),x)-v.begin())

double xa, ya, xb, yb;
double calc(double y) {
	return sqrt(xa*xa + (ya - y)*(ya - y)) + sqrt(xb*xb + (yb - y)*(yb - y));
}

int main() {
	ios::sync_with_stdio(0);
	cin.tie(0);
	cin >> xa >> ya >> xb >> yb;
	double ymin=0, ymax=1000;
	rep(i,100000) {
		double a, b;
		a = (ymin * 2 + ymax) / 3;
		b = (ymin + ymax * 2) / 3;
		if (calc(a)<calc(b)) {
			ymax = b;
		}
		else {
			ymin = a;
		}
	}

	printf("%.10f\n", ymin);
	return 0;
}