#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;
  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;
}