#include <bits/stdc++.h>
#define int long long
#define f first
#define s second
#define pb push_back
#define endl '\n'
using namespace std;

void solve() {
	int a,b,n;
	cin >> a >> b >> n;
	if (n == 1) {
		cout << 0 << endl;
		return;
	}
	int ans = LLONG_MAX;
	for (int use_a = 1; (1 << use_a) <= n; use_a++) {
		priority_queue<int, vector<int>, greater<int>> p;
		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++) p.push(2);
		while (pr < n) {
			int cur = p.top();
			p.pop();
			pr *= cur + 1;
			pr /= cur;
			use_b += 1;
			p.push(cur + 1);
		}
		// cout << use_a << " " << use_b << " " << a * use_a + b * use_b << endl;
		ans = min(ans, a * use_a + b * use_b);
	} cout << ans << endl;
}

int32_t main() {
	ios_base::sync_with_stdio(0); cin.tie(0);
	int tttt = 1;
//	cin >> tttt;
	while (tttt--) solve();
}