#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(){
  int A, B;
  cin >> A >> B;
  int N;
  cin >> N;
  vector<pair<int, pair<int, int>>> Q = quotient_ranges(N);
  int M = Q.size();
  vector<long long> X(M);
  for (int i = 0; i < M; i++){
    X[i] = Q[i].first;
  }
  reverse(X.begin(), X.end());
  vector<long long> dp(M, INF);
  dp[0] = 0;
  for (int i = 0; i < M; i++){
    int p = lower_bound(X.begin(), X.end(), X[i] * 2) - X.begin();
    for (int j = p; j < M; j++){
      dp[j] = min(dp[j], dp[i] + A + (X[j] - 1) / X[i] * B);
    }
  }
  cout << dp[M - 1] << endl;
}