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.util.*;
// warm-up
public class Main {

	static void solve() {
		Scanner sc = new Scanner(System.in);
		int n=sc.nextInt(),k=sc.nextInt(),x=sc.nextInt(),y=sc.nextInt();
		System.out.println(((n>k ? k : n)*x)+(((n-k>0) ? n-k : 0) *y));
		sc.close();
	}

	public static void main(String args[]) {
		solve();
	}

}
