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: Runtime Error)

public class Main {

    public static void main(String[] args) {
        
        System.out.println(
            new Main().calcCost(
                 toInt(args[0])
                ,toInt(args[1])
                ,toInt(args[2])
                ,toInt(args[3])
            )
        );
    }

    public static int toInt (String a) {
        return Integer.parseInt(a);
    }

    private int calcCost(
          final int n
        , final int k
        , final int x
        , final int y) {
        return x * k + y * (n - k);
    }
}