#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 1000000007;
// constexpr int MOD = 998244353;
constexpr int DY4[]{1, 0, -1, 0}, DX4[]{0, -1, 0, 1};
constexpr int DY8[]{1, 1, 0, -1, -1, -1, 0, 1};
constexpr int DX8[]{0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U>
inline bool chmax(T& a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U>
inline bool chmin(T& a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << fixed << setprecision(20);
  }
} iosetup;

ll p(int x, int n) {
  ll res = 1;
  while (n--) res *= x;
  return res;
}

int main() {
  int a, b, n; cin >> a >> b >> n;
  if (n == 1) {
    cout << 0 << '\n';
    return 0;
  }
  ll ans = LINF;
  for (int c = 1; ; ++c) {
    int x = 1, y = 1;
    while (p(x + 1, c) <= n) ++x;
    while (p(y, c) < n) ++y;
    REP(i, c + 1) {
      if (p(x, i) * p(y, c - i) >= n) chmin(ans, 1LL * a * c + ((x - 1LL) * i + (y - 1LL) * (c - i)) * b);
    }
    if ((1 << c) >= n) break;
  }
  cout << ans << '\n';
  return 0;
}