for (int c0 = 0; c0 <= M; c0 += 1) {
  for (int c1 = 0; c1 <= min(N, c0); c1 += 1) {
    S1(c0, c1);
    S2(c0, c1);
  }
  for (int c1 = c0 + 1; c1 <= N; c1 += 1)
    S2(c0, c1);
  for (int c1 = max(0, N + 1); c1 <= c0; c1 += 1)
    S1(c0, c1);
}