CF 106507Q - Counting Grids Again

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Solve time: 29s
Verified: no

Solution

Sure. Please provide the Codeforces problem statement (or link/name), and I’ll write a detailed editorial in a competitive programming style, including:

Problem restatement / intuition
Key observations
Step-by-step derivation of the algorithm
Proof of correctness
Complexity analysis
Common pitfalls
Reference implementation (if you want, in C++17)