Problem Statement

Takahashi is visiting a shop specializing in cabbage.
The shop sells cabbage for X yen (Japanese currency) per head.
However, if you buy more than A heads of cabbage at once, the (A+1)-th and subsequent heads will be sold for Y yen per head.
(It is guaranteed that $Y \lt X.$ See Sample Input/Output 1 for clarity.)
Print the amount of money needed to buy N heads of cabbage.

Input

Input is given from Standard Input in the following format:
N A X Y

Output

Print the amount of money needed to buy N heads of cabbage (as an integer).

Constraints

• $1 \leq N \leq 10^5$
• $1 \leq A \leq 10^5$
• $1 \leq Y \lt X \leq 100$
• All values in input are integers.

Sample Input 1

5 3 20 15

Sample Output 1

90

You need to pay 20 yen for each of the 1-st through 3-rd heads of cabbage, and 15 yen for each of the 4-th and 5-th heads of cabbage.
Thus, you need to pay a total of 20+20+20+15+15 = 90 yen for the 5 heads of cabbage.

Sample Input 2

10 10 100 1

Sample Output 2

1000

见题面 Input 部分

见题面 Output 部分