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: Accepted)

<?php

fscanf(STDIN, '%d', $n);
fscanf(STDIN, '%d', $k);
fscanf(STDIN, '%d', $x);
fscanf(STDIN, '%d', $y);

$cost = 0;
for ( $i=1; $i<=$n; $i++) {
    if ($i<=$k) {
        $cost += (int)$x;
    } else {
        $cost += (int)$y;
    }
}

echo $cost . "\n";