for (int c0 = 3; c0 < 2 * M; c0 += 1) {
  for (int c1 = c0 / 2 + 2; c1 <= M; c1 += 1)
    for (int c3 = c0 / 2 + 1; c3 < min(c0, c1); c3 += 1)
      S1(c3, c0 - c3, c1);
  for (int c1 = max(1, -M + c0); c1 < (c0 + 1) / 2; c1 += 1)
    S2(c0 - c1, c1);
  for (int c1 = c0 / 2 + 2; c1 <= M; c1 += 1)
    for (int c3 = c0 / 2 + 1; c3 < min(c0, c1); c3 += 1)
      S3(c3, c0 - c3, c1);
}