#include<bits/stdc++.h>
using namespace std;
using ll=long long;
const int N=1e7+5;
ll n, x, y, ans=4e18;
int main() {
//	freopen("dice.in", "r", stdin);
//	freopen("dice.out","w",stdout);
	scanf("%lld %lld %lld", &x, &y, &n);
	if(n==1) {
		puts("0");
		return 0; 
	}
	if((__int128)n*y<=(ll)(1e18))
		ans=min(ans, x+(n-1)*y);

	ll sq=sqrt(n), t=sq+sq-2;
	if(sq*sq<n) t++;
	if(sq*(sq+1)<n) t++;
	if((__int128)t*y<=(ll)(1e18))
		ans=min(ans, x+x+t*y);
	for(ll i=3; i<=__lg(n)+1; i++) {
		priority_queue<ll> q; 
		ll now=1, b=0;
		for(int j=1; j<=i; j++) q.push(-2), now=now*2;
		while(now<n) {
			ll u=-q.top(); q.pop();
			now/=u; now*=u+1; 
			q.push(-(u+1));
			b++;
		}
		ans=min(ans, i*x+(i+b)*y);
	}
	printf("%lld\n", ans);
	return 0;
}