def main():
  from math import inf, 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):
    root = ceil(n ** (1 / i))
    ok, ng = 0, i
    while ng - ok > 1:
      mid = (ok + ng) // 2
      if (root - 1) ** mid * root ** (i - mid) >= n:
        ok = mid
      else:
        ng = mid
    ans = min(ans, (a + b * (root - 2)) * ok + (a + b * (root - 1)) * (i - ok))
    if root == 2:
      break
  print(ans)
if __name__ == '__main__':
  main()