#include <string>
#include <vector>
#include <algorithm>
#include <numeric>
#include <set>
#include <map>
#include <queue>
#include <iostream>
#include <sstream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstring>
#include <cctype>
#include <cassert>
#include <limits>
#include <functional>
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
#if defined(_MSC_VER) || __cplusplus > 199711L
#define aut(r,v) auto r = (v)
#else
#define aut(r,v) __typeof(v) r = (v)
#endif
#define each(it,o) for(aut(it, (o).begin()); it != (o).end(); ++ it)
#define all(o) (o).begin(), (o).end()
#define pb(x) push_back(x)
#define mp(x,y) make_pair((x),(y))
#define mset(m,v) memset(m,v,sizeof(m))
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL
using namespace std;
typedef vector<int> vi; typedef pair<int,int> pii; typedef vector<pair<int,int> > vpii; typedef long long ll;
template<typename T, typename U> inline void amin(T &x, U y) { if(y < x) x = y; }
template<typename T, typename U> inline void amax(T &x, U y) { if(x < y) x = y; }

template<typename Func>
double goldenSectionSearch(double a, double b, Func find_func) {
	const double r = 2 / (3 + sqrt(5.));
	double c = a + r * (b - a), d = b - r * (b - a);
	double fc = find_func(c), fd = find_func(d);
	rep(ii, 100) {
		if(fc < fd) { // '<': maximum, '>': minimum
			a = c; c = d; d = b - r * (b - a);
			fc = fd; fd = find_func(d);
		}else {
			b = d; d = c; c = a + r * (b - a);
			fd = fc; fc = find_func(c);
		}
	}
	return c;
}

int main() {
	int xa; int ya; int xb; int yb;
	while(~scanf("%d%d%d%d", &xa, &ya, &xb, &yb)) {
		double ans = goldenSectionSearch(0, 1000, [=](double y) {
			return -(hypot(xa, y - ya) + hypot(xb, y - yb));
		});
		printf("%.10f\n", ans);
	}
	return 0;
}