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