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