#include <iostream>
#include <iomanip>

using namespace std;

int main()
{
  double x1, y1, x2, y2;
  cin >> x1 >> y1 >> x2 >> y2;

  x1 *= -1;

  const double a = (y1 - y2) / (x1 - x2);
  const double b = y1 - a * x1;

  cout << setprecision( 16 ) << b << endl;  
}