#include <iostream>
#include <unordered_map>
#include <unordered_set>
#include <set>
#include <vector>
#include <numeric>
#include <algorithm>
#include <queue>
#include <string>
#include <random>
#include <array>
#include <climits>
#include <map>
#include <cassert>
#include <stack>
#include <iomanip>
#include <cfloat>
#include <bitset>
#include <fstream>
#include <chrono>

long long int a, b;
std::unordered_map<int, long> cache;
long long int solve(const int n) {
	if (n == 1) return 0;
	if (cache.find(n) != cache.end()) return cache[n];
	long long int min = (n - 1) * b + a;
	for (auto i = 2; i * i <= n; ++i) {
		const auto j = (n + i - 1) / i;
		const auto r = solve(j) + b * (i - 1) + a;
		const auto r2 = solve(i) + b * (j - 1) + a;
		min = std::min({ min, r, r2 });
	}
	return cache[n] = min;
}

int main() {
	int n; std::cin >> a >> b >> n;
	const auto result = solve(n);
	std::cout << result << '\n';
	//std::cerr << cache.size() << '\n';
	//std::cerr << count << '\n';
}