for (int c0 = 0; c0 <= 14; c0 += 1)
  for (int c1 = 0; c1 < n - 14; c1 += 1)
    S1(c0, c1);
for (int c0 = 15; c0 <= n; c0 += 1) {
  for (int c1 = 0; c1 <= 9; c1 += 1)
    S1(c0, c1);
  for (int c1 = 10; c1 < n - 14; c1 += 1) {
    S1(c0, c1);
    S2(c0, c1);
  }
  for (int c1 = n - 14; c1 <= n; c1 += 1)
    S2(c0, c1);
}