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 {

	void run() {
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();
		int k = sc.nextInt();

		long x = sc.nextLong();
		long y = sc.nextLong();

		long sum = 0;
		if (n > k) {
			sum += x * k;
			sum += y * (n - k);
		} else {
			sum += x * n;
		}

		System.out.println(sum);

	}

	public static void main(String[] args) {
		new Main().run();
	}
}