#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
using ll = long long;
const ll C = numeric_limits<int>::max();

inline ll f(int s, int t){ return s * C + t; }

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);

    using ll = long long;
    ll A,B,N; cin >> A >> B >> N;
    using P = pair<ll,ll>;
    priority_queue<P,vector<P>,greater<P>> q;
    q.push({0LL, f(1, 0)});
    unordered_map<ll,ll> mp;
    mp[f(1, 0)] = 0;
    while(!q.empty()) {
        auto [dist, p] = q.top(); q.pop();
        int s = (int)(p / C), t = p % C;
        if(dist != mp[f(s, t)]) continue;

        if(s >= N) {
            cout << dist << endl; return 0;
        }

        if(!mp.count(f(s, s)) || mp[f(s, s)] > dist + A) {
            mp[f(s, s)] = dist + A;
            q.push({dist + A, f(s, s)});
        }

        if(!mp.count(f(s + t, t)) || mp[f(s + t, t)] > dist + B) {
            mp[f(s + t, t)] = dist + B;
            q.push({dist + B, f(s + t, t)});
        }
    }
}