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: Runtime Error)

<?php
 while ($line = fgets(STDIN)) {
    // 配列を生成する
    $arr[] = trim(int($line));
}
   //0:N 1:K 2:X 3:Y　n泊-k泊：　Y円　k泊　　：X円　n泊合計＝k*X+(n-k)*Y
   //制約、整数化
   $sum = $arr[1]*$arr[2]+($arr[0]-$arr[1])*$arr[3];
   echo $sum;
?>