#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
int main(){
	ll a,b,x;
	cin>>a>>b>>x;
	ll mod = x % 3;
	ll sum3 = ((x + 2) / 3) * b;
	ll sum1 =  mod * a + b * (x / 3);
	ll sum2 =  x * a;
	cout<<min({sum1,sum2,sum3})<<endl;
	return 0;
}