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())
print(str(min(N,K)*X+min(0,(N-K))*Y))

N=int(input())
K=int(input())
X=int(input())
Y=int(input())
print(str(min(N,K)*X+max(0,(N-K))*Y))