#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<cctype>
#include<cstdlib>
#include<algorithm>
#include<bitset>
#include<vector>
#include<list>
#include<deque>
#include<queue>
#include<map>
#include<set>
#include<stack>
#include<cmath>
#include<sstream>
#include<fstream>
#include<iomanip>
#include<ctime>
#include<complex>
#include<functional>
#include<climits>
#include<cassert>
#include<iterator>
#include<unordered_map>
#include<unordered_set>
using namespace std;
double xa;
double ya;
double xb;
double yb;
double f(double y) {
	return sqrt(xa*xa + (ya - y)*(ya - y)) + sqrt(xb*xb + (yb - y)*(yb - y));
}
int main() {
	cin >> xa >> ya >> xb >> yb;
	double mint = 0;
	double maxt = 1145141919;
	while ((maxt-mint)>0.0000001&&clock()/(double)(CLOCKS_PER_SEC)<1.5) {
		double sa = maxt - mint;
		sa /= 3.0;
		double mid1 = mint + sa;
		double mid2 = mid1 + sa;
		if (f(mid1) > f(mid2)) {
			mint = mid1;
		}
		else {
			maxt = mid2;
		}
	}
	printf("%.7f\n", mint);
	return 0;
}