from math import * c, d = map(int, input().split()) n, m = map(int, input().split()) k = int(input()) if n * m - k <= 0: print(0) else: print(min(d * (n * m - k), c * ceil((n * m - k) / n), c * ((n * m - k) // n) + d * ((n * m - k) % n) ))