#include <bits/stdc++.h>

#define s second
#define f first
#define ll long long
#define p pair <int, int>

using namespace std;

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    ll n, m, t;
    cin >> n >> m >> t;
    ll l = 0, r = min(n, m) + 1;
    while (r - l > 1) {
        ll mid = (r + l) / 2;
        if (m * n - (m - 2 * mid) * (n - 2 * mid) <= t) l = mid;
        else r = mid;
    }
    cout << l;
    return 0;
}