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.*;
class Main{
  public static void main(String[] arg){
    Scanner stdIn=new Scanner(System.in);
    int N=stdIn.nextInt();
    int K=stdIn.nextInt();
    int X=stdIn.nextInt();
    int Y=stdIn.nextInt();
    int o=N-K;
    if(o>0){
      System.out.println(X*K+o*Y);
    }else{
      System.out.println(N*X);
    }
  }
}
      
    
    