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