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

using ll = long long;

void chmin(ll& a, ll b) {
    if (a > b) a = b;
}

ll my_pow(ll a, ll b) {
    ll res = 1;
    while (b) {
        if (b & 1) res *= a;
        a *= a;
        b >>= 1;
    }
    return res;
}

ll kth_root(ll k, ll n) {
    if (n <= 1 || k == 1) return n;
    ll res = pow(n, 1.0 / k);
    while (my_pow(res, k) > n) --res;
    while (my_pow(res + 1, k) <= n) ++res;
    return res;
}

int main() {
    ll A, B, N; cin >> A >> B >> N;
    if (N == 1) {
        puts("0");
        return 0;
    }
    ll ans = 1e18;
    for (int i = 1; (1ull << (i - 1)) < N; ++i) {
        ll a = kth_root(i, N);
        ll b = a + 1;
        for (int j = 0; j <= i; ++j) {
            if (my_pow(a, j) * my_pow(b, i - j) >= N) {
                chmin(ans, (A + (a - 1) * B) * j + (A + (b - 1) * B) * (i - j));
            }
        }
    }
    cout << ans << endl;
}