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:
N=int(input())
K=int(input())
X=int(input())
Y=int(input())
amount = 0
amount1=0
if K <= N:
    for i in range(1, K + 1):
        amount = amount + X
if K>N:
    for d in range(1, N+1):
        amount=amount+X
for j in range(K+1,N+1):
    amount1=amount1+Y
total=amount+amount1
print(total)