Problem:
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

Solution in PHP (5.6.30) (judge verdict: Accepted)

<?php
fscanf(STDIN, "%d", $need_tomari);
fscanf(STDIN, "%d", $waribiki_day);
fscanf(STDIN, "%d", $normal_pay);
fscanf(STDIN, "%d", $waribiki_pay);


if (($need_tomari < 1 || 10000 < $need_tomari)
    || ($waribiki_day < 1 || 10000 < $waribiki_day)
    || ($normal_pay < 1 || 10000 < $normal_pay)
    || ($waribiki_pay < 1 || 10000 < $waribiki_pay)
    || ($normal_pay < $waribiki_pay)
) {
    exit;
}

$pay = $need_tomari * $normal_pay;
$waribiki = ($need_tomari - $waribiki_day) * ($normal_pay - $waribiki_pay);

if ($need_tomari > $waribiki_day) {
    echo $pay - $waribiki;
} else {
    echo $pay;
}

exit;

?>