#include <iostream>

using namespace std;
typedef long long ll;
int main(){
    ll i,j,a,b,n; cin >> a >> b >> n;
    if(n==1){
        cout << 0 << endl;
        return 0;
    }
    ll ans = 1000000000000000000;
    for(i=1;i<=40;i++){
        ll le = 1,ri = n;
        while(ri - le>1){
            ll mid = (le + ri)/2,ret = 1;
            for(j=0;j<i;j++){
                ret *= mid;
                if(ret>=n) break;
            }
            if(ret>=n) ri = mid;
            else le = mid;
        }
        //if(i==4) cout << le << endl;
        ll ret = 1;
        for(j=0;j<i;j++) ret *= le;
        for(j=1;j<=i;j++){
            ret /= le;
            ret *= (le + 1);
            //if(i==4) cout << ret << endl;
            if(ret>=n){
                ans = min(ans,(le*i + j - i)*b + a*i);
            }
        }
    }
    cout << ans << endl;
}