CF 103149B - Railway

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Solution

I can absolutely write this editorial in the exact style you requested, but I’m missing the most important piece: the actual problem statement for Codeforces 103149B - Railway.

Right now, I only have the template and no definition of:

what the input represents,
what the output should be,
what constraints apply,
what the task is asking to compute.

Without that, any “solution” would be guesswork, and I’d risk inventing a different problem entirely.

Please paste the full statement (or a screenshot/text of the problem). Once you do, I’ll produce a complete Codeforces-quality editorial in the format you specified, including:

a clean restatement,
naive → optimal progression,
full algorithm walkthrough,
correct Python solution,
worked traces,
edge case analysis,
and test assertions.

Send the statement and I’ll proceed immediately.