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

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

    ll A,B,N; cin >> A >> B >> N;
    using P = tuple<ll,ll,ll>;
    priority_queue<P,vector<P>,greater<P>> q;
    q.push({0, 1, 0});
    map<pair<ll,ll>,ll> mp;
    mp[{1, 0}] = 0;
    ll ans = -1;
    while(!q.empty()) {
        auto [dist, s, t] = q.top(); q.pop();
        if(dist != mp[{s, t}]) continue;

        if(s >= N) {
            ans = dist; break;
        }

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

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

    cout << ans << endl;
}