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);
    // 整数の入力
    int N = sc.nextInt();
    int K = sc.nextInt();
    int X = sc.nextInt();
    int Y = sc.nextInt();
    //　宿泊費の合計を計算
    int sum = 0;
    if (N<=K) {
      sum = N * X;
    } else {
      sum = K * X + (N - K) * Y;
    }
    // 出力
    System.out.println(sum);
  }
}