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Score : $100$ points

### Problem Statement

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.

5
3
10000
9000

### Sample Output 1

48000

The accommodation fee is as follows:

• $10000$ yen for the $1$-st night
• $10000$ yen for the $2$-nd night
• $10000$ yen for the $3$-rd night
• $9000$ yen for the $4$-th night
• $9000$ yen for the $5$-th night

Thus, the total is $48000$ yen.

2
3
10000
9000

20000