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.Scanner;

class Main{
	public static void main(String[] args) {
		Scanner stdIn=new Scanner(System.in);
		int n=stdIn.nextInt(),k=stdIn.nextInt(),x=stdIn.nextInt(),y=stdIn.nextInt(),s=0,i;
		for(i=0;i<n&&i<k;i++) s+=x;
		for(i=k;i<n;i++) s+=y;
		System.out.print(s);
	}
}