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.util.Scanner;

public class Main {
	public static void main(String args[]) {
		Scanner scanner = new Scanner(System.in);
       		String[] strStdIn = new String[4];
		int i = 0;
		while (i != 4) {
			strStdIn[i] = scanner.nextLine();
			i++;
		}

		int totalLiving = Integer.parseInt(strStdIn[0]);
		int initialLiving = Integer.parseInt(strStdIn[1]);
		int initialCost = Integer.parseInt(strStdIn[2]);
		int latterCost = Integer.parseInt(strStdIn[3]);
		
		int latterLivingDate = totalLiving - initialLiving;

		int totalCost = 0;

		if ( (initialLiving == 0) || (latterLivingDate <= 0) ) {
			totalCost = initialCost * totalLiving;
		} else {
			totalCost = latterCost * latterLivingDate + initialCost * (totalLiving - latterLivingDate);
		}
		
		String returnString = Integer.toString(totalCost);

        	System.out.println(totalCost);
    }
}