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
import java.util.*;
public class Main{
  public static void main(String[] args){
    int[] a = new int[4];
    Scanner s = new Scanner(System.in);
    for(int i=0; i<4; i++){
      a[i] = Integer.parseInt(s.nextLine());
    }
    int w = Math.max(a[0]-a[1], 0);
    System.out.println(w*a[3]+a[2]*(a[0]-w));
  }
}