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 is given from Standard Input in the following format:

```
$N$ $A$ $X$ $Y$
```

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

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

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.