#include <stdio.h>
int main(){
  double xa,ya,xb,yb;
  double a,b;
  
  scanf("%lf %lf",&xa,&ya);
  scanf("%lf %lf",&xb,&yb);

  xa *=(-1);

  a = (ya-yb)/(xa-xb);
  b = ya-xa*a;
  printf("%lf\n",b);
  
  return 0;

}