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.*;
public class Main {
	public static void main(String[] args){
	  Scanner sc = new Scanner(System.in);
	  // スペース区切りの整数の入力
	  int N= sc.nextInt();
      int K=sc.nextInt();
      int X=sc.nextInt();
      int Y=sc.nextInt();
	  int Sum=0;
		// 出力
      if (N>=K){
      Sum=K*X+(N-K)*Y;
      }else{
      Sum=N*X;
      }
      System.out.println(Sum);
   }
}