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 {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);

		int n = s.nextInt();
		int k = s.nextInt();
		int x = s.nextInt();
		int y = s.nextInt();
		int i = 0;
		int sum = 0;

		for (i = 0; i < n; i++) {
			if (i < k) {
				sum = sum + x;
			} else if (i >= k) {
				sum = sum + y;
			}
		}
		System.out.println(sum);
	}

}