#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i,n) for (int i = 0; i < n; i++)
#define rrep(i,n) for (int i = n-1; i >= 0; i--)
#define rep2(i,a,b) for (int i = a; i < b; i++)
#define rrep2(i,a,b) for (int i = a-1; i >= b; i--)
#define rep3(i,a,b,c) for (int i = a; i < b; i+=c)
#define rrep3(i,a,b,c) for (int i = a-1; i >= b; i-=c)
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
template<class T> bool chmax(T &a, T b){if (a < b){a = b;return true;} else return false;}
template<class T> bool chmin(T &a, T b){if (a > b){a = b;return true;} else return false;}
template<class T> T mypow(T x, ll y){if (y == 0) return 1;T p = pow(x,y/2);if (y%2 == 0) return p*p;else return p*p*x;}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    ll a,b,n; cin >> a >> b >> n;
    if (n == 1){
        cout << 0 << endl;
        return 0;
    }
    ll ans = 1LL<<60;
    rep2(i,1,31){
        ll cnt = a*i, N = n;
        rep(j,i){
            ll l = 0;
            if (i-j != 1) while (mypow(l,i-j) < N) l++;
            else l = N;
            N = (N+l-1)/l;
            cnt += (l-1)*b;
        }
        chmin(ans,cnt);
    }
    cout << ans << endl;
}