#include <iostream>
#include <cmath>
#include <set>
#include <vector>
using namespace std;
int main ()
{
    long long n,m,t;
    cin >> n >> m >> t;
    long long l = 0, r = min(n,m) / 2;
    long long sr;
    while (r > l + 1)
    {
        sr = (l + r) / 2;
        if (2 * sr * m + 2 * sr * n - sr * sr * 4 >= t)
            r = sr;
        else
            l = sr;
    }
    cout << l;
    return 0;
}