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

constexpr long long inf = 1001001001001001001;

int main() {
    long long a,b,n;
    cin >> a >> b >> n;
    if(n == 1) {
        cout << 0 << endl;
        return 0;
    }
    long long ans = inf;
    for(int i = 1; i <= 30; i++) {
        ans = min(ans,(a+b)*i+max(0ll,n-(1 << i)+(1 << (i-1))-1)/(1 << (i-1))*b);
    }
    cout << ans << endl;
}