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

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