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