#include <bits/stdc++.h>
using namespace std;

void solve() {
	int a, b, n;
	cin >> a >> b >> n;
	// if your lucky
	if (n == 1) {
		cout << 0 << endl;
		return;
	}
	// temp answer
	int answer = a + (n - 1) * b;
	// start iterating
	for (int use_a = 2; (1 << (use_a - 1)) <= n; use_a++) {
		priority_queue<int, vector<int>, greater<int>> pq;
		int pr = 1;
		int use_b = use_a;
		for (int i = 1; i <= use_a; i++) pr *= 2;
		for (int i = 1; i <= use_a; i++) pq.push(2);
		while (pr < n) {
			int cur = pq.top();
			pq.pop();
			pr *= cur + 1;
			pr /= cur;
			use_b += 1;
			pq.push(cur + 1);
		}
		// looking for min
		answer = min(answer, a * use_a + b * use_b);
	}
	// couting final answer
	cout << answer << endl;
}

int main() {
	ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
	solve();

	return 0;
}