import math, strutils, sequtils
var a, b: int
(a, b) = stdin.readline.split.map parseInt
var n = stdin.readline.parseInt
if n == 1:
  echo 0
  quit 0
var ans = high(int)
for i in 1..n:
  var root = int(ceil(pow(float(n), 1 / i)))
  var
    ok = 0
    ng = i
  while ng - ok > 1:
    var mid = (ok + ng) div 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
echo ans