#include<bits/stdc++.h>
using namespace std;
using ll=long long;
int main() {
	ll N, C, V;
	cin >> C >> V >> N;
    if(N==1)cout << 0 << endl,exit(0);
	ll ret = 1LL << 61;
	for(ll X = 1;( 1LL << X )<= 2LL * N; X++) {
		ll a = pow(N + 1.0e-9, 1.0 / X);
		ll Y = (log(N) - X * log(a)) / (log(a + 1) - log(a)) + 1 - 1.0e-9;
		ll cost = X * C + (X * (a - 1) + Y) * V;
		if(ret > cost) ret = cost;
	}
	cout << ret << endl;
	return 0;
}