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-----
5
3
10000
9000

-----Sample Output-----
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.

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