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.Scanner;
 
public class Main {
 
	public static void main(String[] args) {
 
 
		Scanner sc = new Scanner(System.in);
 
		int N = sc.nextInt();
		int K = sc.nextInt();
		int X = sc.nextInt();
		int Y = sc.nextInt();
 
		int ret = 0;
		for(int i=0; i<N; i++){
			if (i<K){
				ret = ret+X;
			}
			else {
				ret = ret+Y;
			}
		}
 
 
 
 
 
		System.out.println(ret);
 
 
	}
}