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;

class Main{



	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);			//文字の入力
		int n = sc.nextInt();
		int k = sc.nextInt();
		int x = sc.nextInt();
		int y  =sc.nextInt();
				

		int ans = 0;
		for(int i = 0;i < k;i++){
			if(i >= n){
				break;
			}
			ans += x;
		}
		
		for(int i = 0;i < n-k;i++){
			ans += y;
		}
		System.out.println(ans);
	}
}