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:
import sys 

N = int(sys.stdin.readline())
K = int(sys.stdin.readline())
X = int(sys.stdin.readline())
Y = int(sys.stdin.readline())

#X* K + Y * (N-K)
# if N > K

#X * K
# if N < K

if N > K : 
    print X* K + Y *(N-K)
else:
    print X*N