#include<bits/stdc++.h>
using namespace std;
#define int long long
int n,a,b,ans;
signed main() {
	cin>>a>>b>>n;
	if(n==1) {
		cout<<0;
		return 0;
	}
	ans=a+(n-1)*b;
	for(int i=2;(1ll<<(i-1))<=n;i++) {
		priority_queue<int,vector<int>,greater<int> > p;
		int j=i,now=1;
		for(int k=1;k<=i;k++) {
			now*=2;
			p.push(2);
		}
		while(now<n) {
			int man=p.top();
			p.pop();
			now=now*(man+1)/man;
			j++;
			p.push(man+1);
		}
		ans=min(ans,a*i+b*j);
	}
	cout<<ans;
	return 0;
}
//log2^2