int convexity_n4(vector<point> a) {
  sort(all(a)); //sort by increasing of x, if equal: by y

  int ans = 0;
  forn(i, a.size()) {
    vector<point> b(a.begin() + i + 1, a.end());
    sort(all(b), cmp(a[i])); //sort by relative angle to a[i]

    b.insert(b.begin(), a[i]);

    if ((int)b.size() <= ans)
      continue;

    memset(z, 0, sizeof(z));
    for (int j = 1; j < (int)b.size(); j++) {
      z[0][j] = 2;
      forn(t, j)
        if (z[t][j]) {
          ans = max(ans, z[t][j]);
          for (int k = j + 1; k < (int)b.size(); k++)
            if (vec(b[t], b[j], b[k]) > 0)
              z[j][k] = max(z[j][k], z[t][j] + 1);
        }
    }
  }

  return ans;
}