kak

#include <iostream>

using namespace std;

typedef long long ll;

int main() {

    ll n, m, t;

    cin >> n >> m >> t;

    if (n < m)
        swap(n, m);

    ll r = n - m;

    ll h, w;

    if (m % 2 == 0)
        h = 2;
    else h = 1;

    w = h + r;

    while (n * m - w * h > t) {
        w += 2;
        h += 2;
    }

    cout << (n - w) / 2;

    return 0;
}