Hi, Codeforces!

On Oct/09/2026 17:35 (Moscow time) we will host Codeforces Round 1130 (Div. 1 + Div. 2). The round is combined and rated for all participants.

You will be given 8 problems and 2 hours 30 minutes to solve them. One of the problems is interactive, so please read the guide for interactive problems if you are not familiar with them.

The problems were authored and prepared by kaleido and shiomi. Thanks to:

• orbit for coordinating the round and rejecting our favourite problem twice;
• wavelet, bitsetter, caramel_tea, segtree_beats, neon_tetra, ricecake and lemonade for testing;
• MikeMirzayanov for the great Codeforces and Polygon systems.

Score distribution: 250 – 500 – 1000 – 1500 – 2000 – 2500 – 3000 – 3500.

UPD: the editorial will be published right after system testing.

Thank you for participating! Hints come before the tutorials, so open them one at a time if you want to keep thinking.

2304A — Odd One Out

2304B — Monocarp and Coins

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A short note, because I keep seeing this in my students' code. To process all pairs (mask, submask), people loop over all 2ⁿ values twice and check (s & mask) == s. That is 4ⁿ iterations. The standard idiom:

for (int mask = 0; mask < (1 << n); mask++) {
    for (int s = mask; s > 0; s = (s - 1) & mask) {
        // s is a non-empty submask of mask
    }
}

Every bit is in one of three states: not in mask, in mask but not in s, or in both. So the inner loop runs 3ⁿ times in total: for n = 18 that is about 3.9·10⁸ instead of 6.9·10¹⁰.

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