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;

public class Main
{
	public static void main(String[] args)
	{	
		Scanner scan = new Scanner(System.in);
		long n , k , x , y;
		n = scan.nextLong();
		k = scan.nextLong();
		x = scan.nextLong();
		y = scan.nextLong();
		if (n >= k)
			System.out.println(k * x + (n - k) * y);
		else
			System.out.println(n * x);
	}
	
}



