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;
public class Main{ 
	public static void main(String[] args) {
	Scanner input = new Scanner (System.in);
	int s;
   int n = input.nextInt();
   int k = input.nextInt();
   int x = input.nextInt();
   int y = input.nextInt();
 s=Math.min(n,k) * x +Math.max(n-k,0)*y;
 System.out.println(s);
   
}}