#include <bits/stdc++.h>
using namespace std;

#define INF 1.1e9
#define LINF 1.1e18
#define FOR(i,a,b) for (int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
#define pb push_back
#define pf push_front
#define fi first
#define se second
#define BIT(x,n) bitset<n>(x)
#define PI 3.14159265358979323846

typedef long long ll;
typedef pair<double,double> P;
typedef pair<int,P> PP;

//-----------------------------------------------------------------------------

P p1,p2;

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	cin>>p1.fi>>p1.se>>p2.fi>>p2.se;
	p2.fi*=-1;
	double a=(p1.se-p2.se)/(p1.fi-p2.fi);
	double b=p1.se-a*p1.fi;
	printf("%.10f\n",b);

	return 0;
}