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.
n= int(input())
k= int(input())
x= int(input())
y= int(input())
if n <= k:
    print(x*n)
else:
    print(x*k + y*(x-k))
n= int(input())
k= int(input())
x= int(input())
y= int(input())
if n <= k:
    print(x*n)
else:
    print(x*k + y*(n-k))