base_taste, base_cal, max_add_bacon, one_bacon_taste, one_bacon_cal = map(int, input().split())

happy = -10 ** 20

for add_bacon in range(max_add_bacon + 1):
    taste = base_taste + one_bacon_taste * add_bacon
    cal = base_cal + one_bacon_cal * add_bacon
    
    happy = max(taste - cal, happy)

print(happy)