#include<bits/stdc++.h>
#define ll __int128
#define il inline
#define ls p<<1
#define rs p<<1|1
const ll N = 1e5 + 10;
const ll mod = 1e9+7;
const ll inf = 2e18;

using namespace std;

void read(ll &x) {
	x = 0;
	char c = getchar();
	ll f = 0;
	for (;!isdigit(c); c = getchar())
		f |= c == '-';
	for (; isdigit(c); c = getchar())
		x = x*10 + (c ^ '0');
	if (f) x = -x;
}

ll n, x, y, ans = inf;

il bool solve(ll f, ll z) {
	ll a = z / f, b = z % f;
	ll numd = b, numz = f-b;
	ll res = 1; 
	for (ll i = 1; i <= numd; i++) {
		res *= (a+2);
		if (res > n) return 1;
	}
	for (ll i = 1; i <= numz; i++) {
		res *= (a+1);
		if (res > n) return 1;
	}
	return 0;
}


int main() {
//	freopen("dice.in","r",stdin);
//	freopen("dice.out","w",stdout);
	read(x); read(y); read(n);
	ll len = log2((long long)n)+2;
	// O(logn * logn * logn)
	for (ll i = 1; i <= len; i++) { // ??????
		ll l = i, r = n+1, res; 
		while (l <= r) { // ??????
			ll mid = (l + r) >> 1;
			if (solve(i, mid)) {
				r = mid-1;
				res = mid; 
			} else {
				l = mid+1;
			}
		}
		//cout << i << " " << res << endl;
		ans = min(ans, i*x + res*y);
	}
	cout << (unsigned long long)ans << "\n";	
	return 0;
}
/*
1000000000000000000 2312241 2424211
*/