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 stdIn = new Scanner(System.in);
		
		int a = stdIn.nextInt();
		int b = stdIn.nextInt();
		int c = stdIn.nextInt();
		int d = stdIn.nextInt();
		
		int e = (b * c) + (a - b) * d;
		int f = (a * c);
		
		if (a > b) {
			System.out.println(e);
		} else {
			System.out.println(f);
		}
	}
}