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
/*
    Problem URL : http://agc004.contest.atcoder.jp/tasks/agc004_b
12-
    Score  :
    Result :
    Time   : ms
    Memory : KB
 */

ini_set('error_reporting', E_ALL & ~E_NOTICE);

fscanf(STDIN, "%d", $N);
fscanf(STDIN, "%d", $K);
fscanf(STDIN, "%d", $X);
fscanf(STDIN, "%d", $Y);

$ans = 0;
$ans += min($K, $N) * $X;
if ($K < $N) {
    $ans += ($N - $K) * $Y;
}
echo $ans . PHP_EOL;