unhappy friends

uf.py

import sys, os

# unhappy friends
# https://leetcode.com/contest/weekly-contest-206/problems/count-unhappy-friends/
# eg ::: input: [[1, 3, 2], [2, 3, 0], [1, 3, 0], [0, 2, 1]], pairs = [[1, 3], [0, 2]], output: 4

if __name__ == "__main__":
    f_n = int(input("Enter friends ::: "))
    if 2 <= f_n <= 500 and f_n % 2 == 0:
        preferences = []
        pairs       = []
        for i in range(f_n):
            friends = []
            for j in range(1, f_n):
                f = int(input(f"Enter friends of person {i} based on preferences order ::: "))
                if f not in friends and 0 <= f <= f_n-1 and f != i:
                    friends.append(f)
                else:
                    print("[-] either repeated friend in rel list, lower/upper bound issue or found existing person her/him-self in his/her rel list")
                    sys.exit(1)
            preferences.append(friends)

        for k in range(int(f_n/2)):
            p_friends = []
            for l in range(2):
                f = int(input(f"Enter person {l} of pair {k} ::: "))
                if f not in p_friends and 0 <= f <= f_n-1:
                    p_friends.append(f)
                else:
                    print("[-] either repeated friend in one pair or lower/upper bound issue")
                    sys.exit(1)
            pairs.append(p_friends)

        p_tmp = pairs[0]
        for n in range(1, len(pairs)):
            if pairs[n][0] == p_tmp[0] or pairs[n][1] == p_tmp[1]:
                print("[-] found existing person in another pair")
                sys.exit(1)
            else:
                p_tmp = pairs[n]

        count_u_h_f = 0
        for p in pairs:
            f_p = p[0]
            s_p = p[1]
            pref_f_p = preferences[f_p]
            pref_s_p = preferences[s_p]
            is_f_happy = True
            is_s_happy = True
            if pref_f_p[0] != s_p:
                is_f_happy = False
                count_u_h_f += 1
            if pref_s_p[0] != f_p:
                is_s_happy = False
                count_u_h_f += 1

        
        if count_u_h_f != 0:
            print(f"{count_u_h_f} friends are unhappy :-(")
        else:
            print("no one is unhappy :-)")