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: Wrong Answer)

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Scanner;

public class Main {

	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);

		String keyBoard = sc.next();
		int inputSize = keyBoard.length();
		boolean beautifulFlag = true;

		List<String> wordList = new ArrayList<String>();

		for (int i = 0; i < inputSize; i++) {
			wordList.add(keyBoard.substring(i, i+1));

		}

		Collections.sort(wordList);

		for (int i = 0; i < wordList.size(); i++) {
			String word = wordList.get(0);

			int wordCount = wordList.lastIndexOf(word) + 1;

			if (wordCount % 2 != 0) {
				System.out.println("No");
				beautifulFlag = false;
				break;
			}

			for (int j = 0; j < wordCount; j++) {
				wordList.remove(0);
			}
		}

		if (beautifulFlag) {
			System.out.println("Yes");

		}

		sc.close();
	}
}