Problem: 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

Solution (in PYTHON):

Solution:
def main():                                                                     
  n, k, x, y = int(raw_input()), int(raw_input()), int(raw_input()), int(raw_input())
  total = 0                                                                     
  for i in xrange(1, n+1):                                                      
    total += x if i <= k else y                                                 
  print(total)                                                                  
                                                                                
main()