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.

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.

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).