#include<bits/stdc++.h>

using namespace std;
using ll = long long;


int main()
{
    int a, b, x; cin >> a >> b >> x;

    int nax = 1e9;

    for(int i = 0;  i < 100; i++) {
        for(int j = 0; j < 100; j++) {
            int wata = i * 1000 + j * 500;
            if(wata < x)continue;
            nax = min(i * a + j * b, nax);
        }
    }
    cout << nax << endl;
}
