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


int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	// iad
	int a, b, n;
	cin >> a >> b >> n;
	if (n == 1) {
		cout << 0 << endl;
		return 0;
	}
	// temp answer
	int ans = a + (n - 1) * b;
	for (int use_1 = 2; (1 << (use_1 - 1)) <= n; use_1++) {
		priority_queue<int, vector<int>, greater<int>> pq;
		int duck = 1;
		int use_2 = use_1;
		for (int i = 1; i <= use_1; i++) duck *= 2;
		for (int i = 1; i <= use_1; i++) pq.push(2);
		while (duck < n) {
			int cur = pq.top();
			pq.pop();
			duck *= cur + 1;
			duck /= cur;
			use_2 += 1;
			pq.push(cur + 1);
		}
		
		ans = min(ans, a * use_1 + b * use_2);
	}
	cout << ans << endl;
	return 0;
}