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.io.IOException;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws IOException {
		try (Scanner s = new Scanner(System.in)) {
			int n = s.nextInt();
			int k = s.nextInt();
			int x = s.nextInt();
			int y = s.nextInt();
			int ax = k, bx = 0;
			if (n > k) {
				bx = n - k;
			} else {
				ax = n;
			}
			System.out.println(ax * x + bx * y);
		}
	}
}
