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 Kotlin (1.0.0) (judge verdict: Accepted)

/**
 * @author karayuu
 */
fun main(args: Array<String>) {
    val n = readLine()!!.toInt()
    val k = readLine()!!.toInt()
    val x = readLine()!!.toInt()
    val y = readLine()!!.toInt()

    if (n <= k) {
        print((1..n).sumBy { x })
    } else {
        print((1..k).sumBy { x } + ((k + 1)..n).sumBy { y })
    }
}