t = int(input())
x, a = map(int, input().split())
y, b = map(int, input().split())
ans = 2 * 10 ** 7
for cb in range(2 * 10 ** 7):
    rest = t - (-b * cb)
    if rest < 0:
        continue
    ans = min(ans, y * cb + rest // a * x + rest % a)
print(ans)