题解 | 特殊城市

import sys
from collections import defaultdict

n, d, t = int(sys.stdin.readline()), defaultdict(int), 0
for _ in range(n):
    s, c = sys.stdin.readline().split()
    d[(s[:2],c)] += 1
for (a,b) in d.keys():
    if a<b and (b,a) in d.keys():
        t+=d[(a,b)]*d[(b,a)]
print(t)