Score : 100 points

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