AtCoder Beginner Contest 402 D

https://atcoder.jp/contests/abc402/tasks/abc402_d

2点の番号を足してNで割った余りが同じなら平行です。

// Square Price
#![allow(non_snake_case)]


//////////////////// library ////////////////////

fn read<T: std::str::FromStr>() -> T {
    let mut line = String::new();
    std::io::stdin().read_line(&mut line).ok();
    line.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>().split_whitespace()
            .map(|e| e.parse().ok().unwrap()).collect()
}


//////////////////// process ////////////////////

type Node = usize;
type Edge = (Node, Node);

fn read_edge() -> Edge {
    let v: Vec<Node> = read_vec();
    (v[0] - 1, v[1] - 1)
}

fn read_input() -> (usize, Vec<Edge>) {
    let v: Vec<usize> = read_vec();
    let (N, M) = (v[0], v[1]);
    let edges: Vec<Edge> = (0..M).map(|_| read_edge()).collect();
    (N, edges)
}

fn F(N: usize, edges: Vec<Edge>) -> usize {
    let M = edges.len();
    let mut counter: Vec<usize> = vec![0; N];
    for (A, B) in edges.into_iter() {
        counter[(A+B)%N] += 1
    }
    
    M * (M-1) / 2 - counter.into_iter().map(|n| (n*n-n) / 2).sum::<usize>()
}

fn main() {
    let (N, edges) = read_input();
    println!("{}", F(N, edges))
}