#include<bits/stdc++.h>
using namespace std;
using ll=long long;
int main(){
	ll a,b,n;cin>>a>>b>>n;
	ll x=1,y=0;
	ll ans=0;
	while(n-x>0){
		if(x+y>=n){
			ans+=b;
			break;
		}
		if((a+b)*y<=b*x){
			ans+=a+b;
			x*=2;
			y=x/2;
		}
		else{
			ans+=b;
			x+=y;;
		}
	}
	cout<<ans<<endl;
}