def main():
  from math import inf, floor, ceil
  A, B = map(int, input().split())
  N = int(input())
  if N == 1:
    print(0)
    exit()
  ans = inf
  for i in range(1, N + 1):
    a, b = floor(N ** (1 / i)), ceil(N ** (1 / i))
    powa, powb = [1], [1]
    for j in range(i):
      powa.append(powa[-1] * a)
      powb.append(powb[-1] * b)
    for j in range(i + 1):
      if powa[i] * powb[i - j] >= N:
        ans = min(ans, (A + B * (a - 1)) * j + (A + B * (b - 1)) * (i - j))
    if b == 2:
      break
  print(ans)
if __name__ == '__main__':
  main()