#include <iostream>
#include <algorithm>

using namespace std;

int main()
{
    long long a, b, left;
    cin >> a >> b >> left;

    long long l = 0;
    long long r = max(a, b) / 2 + 1;

    while (r - l > 1) {
        long long m = (l + r) / 2;
        long long y = a * b - (a - 2 * m) * (b - 2 * m);
        if (y > 0 && y <= left && a * b - y > 0)
            l = m;
        else
            r = m;
    }

        cout << l << endl;
}