#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic pop
using namespace std;
using namespace atcoder;
#define rep(i,n)for (int i = 0; i < int(n); ++i)
#define rrep(i,n)for (int i = int(n)-1; i >= 0; --i)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
template<class T> void chmax(T& a, const T& b) { a = max(a, b); }
template<class T> void chmin(T& a, const T& b) { a = min(a, b); }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  ll a, b, n;
  cin >> a >> b >> n;
  if (n == 1) {
    cout << 0 << '\n';
    return 0;
  }
  ll ans = a + (n - 1) * b;
  for(int cnt = 2; cnt <= 30; cnt++) {
    ll l = 1;
    auto p = [](int x, int k) {
      ll res = 1;
      rrep(_, k) res *= x;
      return res;
    };
    while(p(l + 1, cnt) <= n) l++;
    int r = 0;
    while(p(l, cnt - r) * p(l + 1, r) < n) r++;
    ll tot = l * (cnt - r) + (l + 1) * r;
    chmin(ans, tot * b + cnt * (a - b));
  }
  cout << ans << '\n';
}