import java.util.*;
public class taksi
{
 public static void main(String args[])
 { Scanner in = new Scanner(System.in);
  int a = in.nextInt();
int a1 = in.nextInt();
int b = in.nextInt();
int b1 = in.nextInt();
int g = 0;
      for(int i = b;i>a;i=i-b1)
      {
          a=a+a1;
          if(i>a)
         g = a;
          }
          System.out.println(" kelisken bagasy :"+g);
 }
}