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 Scala (2.11.7) (judge verdict: Accepted)

  // A - 高橋君とホテルイージー / Tak and Hotels (ABC Edit) https://atcoder.jp/contests/abc044/tasks/abc044_a

  import io.Source

  object Main extends App {
    val xs = Source.stdin.getLines()
    val n = xs.next().toInt
    val k = xs.next().toInt
    val x = xs.next().toInt
    val y = xs.next().toInt

    println(solve(n, k, x, y))

    def solve(n: Int, k: Int, x: Int, y: Int): String = {
      if (n >= k) {
        (x * k + y * (n - k)).toString
      } else {
        (x * n).toString
      }
    }
  }