def main():
  from math import inf, ceil
  a, b = map(int, input().split())
  n = int(input())
  if n == 1:
    exit(print(1))
  ans = inf
  for i in range(1, n + 1):
    p = ceil(n ** (1 / i))
    ans = min(ans, (a + b * (p - 1)) * i)
    if p == 2:
      break
  print(ans)
if __name__ == '__main__':
  main()