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Contest: Task: Related: TaskB

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.

Sample Input 1

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.

Sample Input 2

2
3
10000
9000

Sample Output 2

20000