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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

class Main {

	public static void main(String[] args) throws Exception {

		try {

			BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

			int howManyNights = Integer.parseInt(br.readLine());
			int firstChargeDay = Integer.parseInt(br.readLine());
			int firstCharge = Integer.parseInt(br.readLine());
			int subsequentCharge = Integer.parseInt(br.readLine());

			int result = getSumCharge(howManyNights, firstChargeDay, firstCharge, subsequentCharge);

			System.out.println(result);

		} catch (IOException | NumberFormatException e) {
			e.printStackTrace();
			System.exit(0);
		} catch (Exception e) {
			e.printStackTrace();
			System.exit(0);
		}

	}

	public static int getSumCharge(int howManyNights, int firstChargeDay, int firstCharge, int subsequentCharge) {
		int sumCharge = 0;

		for(int i = 1; i <= howManyNights; i++) {
			if (i <= firstChargeDay) {
				sumCharge += firstCharge;
			} else {
				sumCharge += subsequentCharge;
			}
		}

		return sumCharge;
	}
}