D - Line Crossing https://atcoder.jp/contests/abc402/tasks/abc402_d

abc402_d.py

from collections import defaultdict

N, M = map(int, input().split())
d = defaultdict(int)
for _ in range(M):
    A, B = map(int, input().split())
    d[(A + B) % N] += 1
ans = M * (M - 1) // 2
for _, v in d.items():
    ans -= v * (v - 1) // 2
print(ans)