Tak and Hotels (ABC Edit)

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

inputFormat

Input

The input is given from Standard Input in the following format:

N K X Y

outputFormat

Output

Print Tak's total accommodation fee.

Examples

Input

5 3 10000 9000

Output

48000

Input

2 3 10000 9000

Output

20000

样例

2
3
10000
9000

20000