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 sc = new Scanner(System.in);
		int span = sc.nextInt();
		int period = sc.nextInt();
		int cost1 = sc.nextInt();
		int cost2 = sc.nextInt();
		int total = 0;
		
		if((span - period) >= 0)
		total = period * cost1 + (span - period) * cost2;
		else 
		total = span * cost1;
		System.out.println(total);
	}
}