CSES 1668 Building Teams - Input Format

CSES uses standard competitive programming I/O format. No fancy JSON or API calls. First line: two integers $n$ and $m$, the number of pupils and the number of friendships. Next $m$ lines: each contains two integers $a$ and $b$ meaning pupils $a$ and $b$ are friends.

You read from stdin and write to stdout. Build the adjacency list yourself from the edge pairs. Pupils are numbered $1$ to $n$, not $0$ to $n-1$. Remember to size your arrays so.