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){
	int N,K,X,Y;

	Scanner sc = new Scanner(System.in);
	N = sc.nextInt();
	K = sc.nextInt();
	X = sc.nextInt();
	Y = sc.nextInt();

	int sum = 0;

	if(N>K){
	    sum =K*X +(N -K)*Y;
	}else{
	    sum = N*X;
	}

	System.out.println(sum);
    }
}
