// Source: https://usaco.guide/general/io

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

int main() {
	// yeeettt
	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 answer = a + (n - 1) * b;
	// iterate
	for (int use1 = 2; (1 << (use1 - 1)) <= n; use1++) {
		// dedlaratiom
		priority_queue<int, vector<int>, greater<int>> pq;
		int duck = 1;
		int use2 = use1;
		for (int i = 1; i <= use1; i++) duck *= 2;
		for (int i = 1; i <= use1; i++) pq.push(2);

		// reiterate
		while (duck < n) {
			int now = pq.top();
			pq.pop();
			duck *= now + 1;
			duck /= now;
			use2 += 1;
			pq.push(now + 1);
		}

		// find the smallest valid one
		answer = min(answer, a * use1 + b * use2);
	}

	// final nail to the cofin
	cout << answer << endl;

	return 0;
}