#include <iostream>
#include <algorithm>
using namespace std;
using ll=long long;
ll N, C, V;
int main() {
	cin >> C >> V >> N;
    if(N==1)cout << 0 << endl,exit(0);
	ll ret = 1LL << 62;
	for (ll i = 1; i <= 30; i++) {
		bool flag = false;
		for (ll j = 1; !flag; j++) {
			for (ll k = i; k >= 1; k--) {
				long long mul = 1;
				for (ll l = 0; l < k; l++) mul *= j;
				for (ll l = k; l < i; l++) mul *= j + 1;
				if (mul >= N) {
					ret = min(ret, (i * j - k) * V + C * i);
					flag = true; break;
				}
			}
		}
	}
	cout << ret << endl;
	return 0;
}