Problem: 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

Solution (in JAVA):

Solution:
import java.util.Scanner;

public class Main {
	private static Scanner scanner = new Scanner(System.in);

	public static void main(String[] args) {
		int N = scanner.nextInt();
		int K = scanner.nextInt();
		int X = scanner.nextInt();
		int Y = scanner.nextInt();
		int a = (N - K) * (X - Y);
		if (a < 0) a = 0;
		System.out.println(N * X - a);
	}
}