#include<cstdio>
#include<cstring>
#include<cctype>
#include<cmath>
#include<climits>
#include<algorithm>
#include<vector>
#include<valarray>
#include<string>
#include<set>
#include<map>
#include<stack>
#include<queue>
#include<iostream>
#include<iomanip>
using namespace std;
   
   
#define rep(i,n) for(int i=0;i<n;i++)
#define REP(i,n) for(int i=1;i<=n;i++)
#define drep(i,n) for(int i=n-1;i>=0;i--)
#define DREP(i,n) for(int i=n;i>0;i--)
#define Rep(i,m,n) for(int i=m;i<n;i++)
#define ReP(i,m,n) for(int i=m;i<=n;i++)
#define scani(a) scanf("%d",&a)
#define scand(a) scanf("%lf",&a)
#define scans(s) scanf("%s",s)
#define readi(a) int a;scanf("%d",&a)
#define printi(a) printf("%d",a)
#define prints(s) printf("%s",s)
#define even(n) (((n)+1)%2)
#define odd(n) ((n)%2)
#define sg(x) ((x)?1:0)
#define LF printf("\n")
#define SPACE printf(" ")
#define pb push_back
#define mp make_pair
#define bs binary_search
#define all(a) (a).begin(),(a).end()
#define forall(itr,a) for(auto itr=(a).begin();itr!=(a).end();++itr)
   
typedef long long LL;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef pair<double,double> pdd;
typedef pair<int,int> pii;
   
const double pi = acos(-1.0);
double rad(double t){return t*pi/180.0;}
double deg(double d){return d*180.0/pi;}
template<typename T>T GCD(T x,T y){return y?GCD(y,x%y):x;}
template<typename T>T LCM(T x,T y){return x/GCD(x,y)*y;}
template<class T>ostream& operator<<(ostream& os,const set<T>& s){os<<"{";for(auto itr=s.begin();s.end() != itr;++itr){if(itr!=s.begin())os<<',';os<<*itr;}os<<"}";return os;}
template<class S,class T>ostream& operator<<(ostream& os,const pair<S,T>& p){os<<'('<<p.first<<','<<p.second<<')';return os;}
const int MOD = (int)pow(10,9) +7;

int main(){
  double x_a,x_b,y_a,y_b;
  cin >> x_a >> y_a >> x_b >> y_b ;
  x_a = - x_a;
  printf("%.6lf",y_a-x_a*((y_a-y_b)/(x_a-x_b)));
  return 0;
}