t = int(input())
x, a = map(int, input().split())
y, b = map(int, input().split())

x = min(x, a)
ans = 1 << 60
for i in range(10 ** 7 + 1):
    d = t + b * i
    if d < 0:
        continue
    j = d // a * x + d % a
    ans = min(ans, i * y + j)
print(ans)