Problem: There is a hotel with the following accommodation fee:

* X yen (the currency of Japan) per night, for the first K nights
* Y yen per night, for the (K+1)-th and subsequent nights



Tak is staying at this hotel for N consecutive nights. Find his total accommodation fee.

Constraints

* 1 \leq N, K \leq 10000
* 1 \leq Y < X \leq 10000
* N,\,K,\,X,\,Y are integers.

Input

The input is given from Standard Input in the following format:


N
K
X
Y


Output

Print Tak's total accommodation fee.

Examples

Input

5
3
10000
9000


Output

48000


Input

2
3
10000
9000


Output

20000

Solution (in JAVA):

Solution:
import java.util.Scanner;

class Main{
 public static void main(String[] args){
   Scanner sc=new Scanner(System.in);
   
   int N=sc.nextInt();
   int K=sc.nextInt();
   int X=sc.nextInt();
   int Y=sc.nextInt();
   
   if(N<=K){
     System.out.println(N*X);
   }else if(N>K){
     System.out.println(K*X+(N-K)*Y);
   }
 }
}