#include <bits/stdc++.h>
using namespace std;
#include <atcoder/modint>
using namespace atcoder;
//using mint = modint998244353;
using mint = modint1000000007;
using ll = long long;
using ld = long double;
using pll = pair<ll, ll>;
using tlll = tuple<ll, ll, ll>;
constexpr ll INF = 1LL << 60;
template<class T> bool chmin(T& a, T b) {if (a > b) {a = b; return true;} return false;}
template<class T> bool chmax(T& a, T b) {if (a < b) {a = b; return true;} return false;}
ll safemod(ll A, ll M) {return (A % M + M) % M;}
ll divfloor(ll A, ll B) {if (B < 0) {return divfloor(-A, -B);} return (A - safemod(A, B)) / B;}
ll divceil(ll A, ll B) {if (B < 0) {return divceil(-A, -B);} return divfloor(A + B - 1, B);}
template<class T> void unique(vector<T> &V) {V.erase(unique(V.begin(), V.end()), V.end());}
template<class T> void sortunique(vector<T> &V) {sort(V.begin(), V.end()); V.erase(unique(V.begin(), V.end()), V.end());}
#define FINALANS(A) do {cout << (A) << '\n'; exit(0);} while (false)
template<class T> void printvec(vector<T> &V) {int _n = V.size(); for (int i = 0; i < _n; i++) cout << V[i] << (i == _n - 1 ? "" : " ");cout << '\n';}
template<class T> void printvect(vector<T> &V) {for (auto v : V) cout << v << '\n';}
template<class T> void printvec2(vector<vector<T>> &V) {for (auto &v : V) printvec(v);}

int main()
{
  // https://sugarknri.hatenablog.com/entry/2017/09/13/130850

  ll A, B, N;
  cin >> A >> B >> N;

  if (N == 1)
    FINALANS(0);

  ll ans = A * 1 + B * (N - 1);
  for (ll k = 2; k < 40; k++)
  {
    ll b = 1;
    for (;; b++)
    {
      ll tmp = 1;
      for (ll i = 0; i < k; i++)
      {
        tmp *= b;
      }
      if (tmp > N)
        break;
    }
    b--;

    for (ll i = 0; i <= k; i++)
    {
      ll mul = 1;
      for (ll j = 0; j < i; j++)
      {
        mul *= b + 1;
      }
      for (ll j = i; j < k; j++)
      {
        mul *= b;
      }

      if (mul >= N)
      {
        ll s = 0;
        for (ll j = 0; j < i; j++)
        {
          s += b;
        }
        for (ll j = i; j < k; j++)
        {
          s += b - 1;
        }
        ll tmp = A * k + B * s;
        if (chmin(ans, tmp))
        {
          //cerr << k << " " << i << " " << b << " " << mul << " " << s << endl;
        }
      }
    }
  }
  cout << ans << endl;
}