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 PHP7 (7.0.15) (judge verdict: Wrong Answer)

<?php
/*
  http://abc044.contest.atcoder.jp/tasks/abc044_b
*/

function is_even($value) {
  return $value % 2 === 0 ? true : false;
}

$stdin = trim(fgets(STDIN));
$str_hash = [];

foreach (str_split($stdin) as $key => $value) {
  if (isset($str_hash[$value])) {
    $str_hash[$value] += 1;
  } else {
    $str_hash[$value] = 1;
  }
}

$is_beautiful = true;
foreach ($str_hash as $key => $value) {
  if (!is_even($value)) {
    $is_beautiful = false;
    break;
  }
}

echo $is_beautiful ? "Yes" : "No";

?>