#include <bits/stdc++.h>
using namespace std;
const long long INF = 1000000000000000;
vector<pair<int, pair<int, int>>> quotient_ranges(int N){
	vector<pair<int, pair<int, int>>> ans;
	for (int i = 1; i * i <= N; i++){
		ans.push_back(make_pair(N / i, make_pair(i, i)));
	}
	for (int i = N / ((int) sqrt(N) + 1); i >= 1; i--){
		ans.push_back(make_pair(i, make_pair(N / (i + 1) + 1, N / i)));
	}
	return ans;
}
int main(){
  long long A, B;
  cin >> A >> B;
  int N;
  cin >> N;
  if (N == 1){
    cout << 0 << endl;
  } else {
    long long ans = INF;
    for (int i = 1; i <= 30; i++){
      int tv = N * 2, fv = -1;
      while (tv - fv > 1){
        int mid = (tv + fv) / 2;
        int Q = mid / i, r = mid % i;
        long long pr = 1;
        bool ok = false;
        for (int j = 0; j < i; j++){
          if (j < r){
            pr *= Q + 2;
          } else {
            pr *= Q + 1;
          }
          if (pr >= N){
            ok = true;
            break;
          }
        }
        if (ok){
          tv = mid;
        } else {
          fv = mid;
        }
      }
      ans = min(ans, A * i + B * tv);
    }
    cout << ans << endl;
  }
}