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

ll a, b;
unordered_map<int, ll> mp;

ll solve(int n) {
	if (n == 1) return 0;
	if (mp.find(n) != mp.end()) return mp[n];
	ll ans = (n - 1) * b + a;
	for (auto i = 2; i * i <= n; ++i) {
		auto temp = (n + i - 1) / i;
		auto tmp1 = solve(temp) + b * (i - 1) + a;
		auto tmp2 = solve(i) + b * (temp - 1) + a;
		ans = min({ ans, tmp1, tmp2 });
	}
	return mp[n] = ans;
}

int main(){
	int n; cin >> a >> b >> n;
	cout << solve(n) << endl;

	return 0;
}