Problem: 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.

Examples

Input

5
3
10000
9000


Output

48000


Input

2
3
10000
9000


Output

20000

Solution (in JAVA):

Solution:
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
    	Scanner sc = new Scanner(System.in);
    	
    	int N = sc.nextInt(), K = sc.nextInt(),
    		X = sc.nextInt(), Y = sc.nextInt(),
    		min = Math.min(N, K), ans = 0;
    	
    	for(int i = 0; i < min; i++)	ans += X;
    	for(int i = K; i < N; i++)	ans += Y;
    	
    	System.out.println(ans);
    }
}