#include <bits/stdc++.h>
// #include <atcoder/all>
using namespace std;
// using namespace atcoder;
typedef int64_t lint;
#define rep(i, n) for(int i=0; i<n; i++)
#define repx(i, l, n) for(int i=l; i<n; i++)
#define all(v) v.begin(), v.end()
#define show(x) cout << #x << ": " << x << endl;
#define list(x) cout << #x << ": " << x << "  ";
#define pb push_back
using vi = vector<lint>;
using vvi = vector<vector<lint>>;
template<class T> inline void vin(vector<T>& v) { rep(i, v.size()) cin >> v.at(i); }
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
template<class T> inline void drop(T x) { cout << x << endl; exit(0); }
template<class T> void vout(vector<T> v) { rep(i, v.size()) { cout << v.at(i) << ' '; } cout << endl; }
constexpr lint LINF = LLONG_MAX/2;
using tint = tuple<lint, lint, lint>;
using vt = vector<tint>;

int main() {
	lint A, B, N;
	cin >> A >> B >> N;
	if (N == 1) drop(0);

	lint a=0, b=0, c=0, x, y, z, ans=LINF;
	priority_queue<tint, vector<tint>, greater<tint>> q;
	q.push({A, -1, -1});
	while (true) {
		tie(x, y, z) = q.top();
		q.pop();
		y *= -1;
		z *= -1;
		if (x > ans) break;
		if (chmax(a, x)) {
			b = 0, c = 0;
		} else if (!chmax(b, y) && !chmax(c, z)) {
			continue;
		}
		if (N >= y) chmin(ans, x+B*((N-y+z-1)/z));
		// list(x)
		// list(y)
		// list(z)
		// show(ans)
		if (B*((N-y+z-1)/z) < A) break;
		if (y == z) {
			q.push({x+B, -(y+z), -z});
		} else {
			q.push({x+B, -(y+z), -z});
			q.push({x+A, -y, -y});
		}
	}
	std::cout << ans << '\n';
}