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 sin = new Scanner(System.in);
    int N = sin.nextInt();
    int K = sin.nextInt();
    int X = sin.nextInt();
    int Y = sin.nextInt();
    
    if(N<=K){
      int rent = X*N;
      System.out.println(rent);
    }else{
      int lent = N-K;
      int lent2 = X*K + lent*Y;
      System.out.println(lent2);
    }
  }  
}