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.*;

public class Main {
	public static void main(String[] args) {
		Scanner scn = new Scanner(System.in);
		int n1 = scn.nextInt();
		int n2 = scn.nextInt();
		int n3 = scn.nextInt();
		int n4 = scn.nextInt();
		int total = 0;
		if(n1>=n2) {
			total+=n2*n3;
			n1-=n2;
		}else {
			total+=n1*n3;
			n1 = 0;
		}
		total+=n1*n4;
		System.out.println(total);
		scn.close();
	}
}