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 Java7 (OpenJDK 1.7.0) (judge verdict: Runtime Error)

public class Main {
	public static void main(String[] args) {
		int[] num = new int[4];
		int money = 0;
		for (int i = 0; i < 4; i++) {
			num[i] = new java.util.Scanner(System.in).nextInt();
		}
		if(num[0]>num[1])
		{
		money = num[1]*num[2]+ num[3]*(num[0]-num[1]);
		}
		else
			money = num[0]*num[2];


		System.out.println(money);
	}
}