def main():
  from math import inf
  a, b = map(int, input().split())
  n = int(input())
  if n == 1:
    exit(print(1))
  ans = inf
  for i in range(1, n + 1):
    ng, ok = 1, n + 1
    while ok - ng > 1:
      mid = (ng + ok) // 2
      if mid ** i >= n:
        ok = mid
      else:
        ng = mid
    tmp = ok ** (i - 1)
    p = (n + tmp - 1) // tmp
    ans = min(ans, (a + b * (ok - 1)) * (i - 1) + (a + b * (p - 1) if p else 0))
    if ok == 2:
      break
  print(ans)
if __name__ == '__main__':
  main()