for (int c0 = p; c0 <= min(m - 1, q); c0 += 1)
  S2(c0);
for (int c0 = m; c0 <= min(n, p - 1); c0 += 1)
  S1(c0);
for (int c0 = max(m, p); c0 <= min(n, q); c0 += 1) {
  S1(c0);
  S2(c0);
}
for (int c0 = max(max(m, n + 1), p); c0 <= q; c0 += 1)
  S2(c0);
for (int c0 = max(max(m, p), q + 1); c0 <= n; c0 += 1)
  S1(c0);