Problem p02997 (python)

Fixed submission:
#!usr/bin/env python3
from collections import defaultdict, deque, Counter, OrderedDict
from bisect import bisect_left, bisect_right
from functools import reduce, lru_cache
from heapq import heappush, heappop, heapify

import itertools, bisect
import math, fractions
import sys, copy

def L(): return sys.stdin.readline().split()
def I(): return int(sys.stdin.readline().rstrip())
def SL(): return list(sys.stdin.readline().rstrip())
def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI1(): return [int(x) - 1 for x in sys.stdin.readline().split()]
def LS(): return [list(x) for x in sys.stdin.readline().split()]
def IR(n): return [I() for _ in range(n)]
def LIR(n): return [LI() for _ in range(n)]
def LIR1(n): return [LI1() for _ in range(n)]
def SR(n): return [SL() for _ in range(n)]
def LSR(n): return [LS() for _ in range(n)]
def LR(n): return [L() for _ in range(n)]

def perm(n, r): return math.factorial(n) // math.factorial(r)
def comb(n, r): return math.factorial(n) // (math.factorial(r) * math.factorial(n-r))

alphabets = "abcdefghijklmnopqrstuvwxyz"
ALPHABETS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"

sys.setrecursionlimit(1000000)
dire = [[1, 0], [0, 1], [-1, 0], [0, -1]]
dire8 = [[1, 0], [1, 1], [0, 1], [-1, 1], [-1, 0], [-1, -1], [0, -1], [1, -1]]
MOD = 1000000007
INF = float("inf")

def main():
    N, K = LI()

    if (N == 2 and K == 0):
        print(1)
        print(1, 2)
        return

    if ((N-1 < 2) or comb(N-1, 2) < K):
        print(-1)
        return
    maxpairs = comb(N-1, 2)

    edges = [[] for _ in range(N)]
    edges[0] = list(range(1, N))

    pairs = list(itertools.combinations(range(1, N), 2))

    for _ in range(maxpairs - K):
        a, b = pairs.pop()
        edges[a].append(b)

    ans = []
    for a in range(N):
        for b in edges[a]:
            ans.append((a+1, b+1))

    print(len(ans))
    for a, b in ans:
        print(a, b)

if __name__ == '__main__':
    main()

Buggy submission (verdict: Runtime Error):
#!usr/bin/env python3
from collections import defaultdict, deque, Counter, OrderedDict
from bisect import bisect_left, bisect_right
from functools import reduce, lru_cache
from heapq import heappush, heappop, heapify

import itertools, bisect
import math, fractions
import sys, copy

def L(): return sys.stdin.readline().split()
def I(): return int(sys.stdin.readline().rstrip())
def SL(): return list(sys.stdin.readline().rstrip())
def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI1(): return [int(x) - 1 for x in sys.stdin.readline().split()]
def LS(): return [list(x) for x in sys.stdin.readline().split()]
def IR(n): return [I() for _ in range(n)]
def LIR(n): return [LI() for _ in range(n)]
def LIR1(n): return [LI1() for _ in range(n)]
def SR(n): return [SL() for _ in range(n)]
def LSR(n): return [LS() for _ in range(n)]
def LR(n): return [L() for _ in range(n)]

def perm(n, r): return math.factorial(n) // math.factorial(r)
def comb(n, r): return math.factorial(n) // (math.factorial(r) * math.factorial(n-r))

alphabets = "abcdefghijklmnopqrstuvwxyz"
ALPHABETS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"

sys.setrecursionlimit(1000000)
dire = [[1, 0], [0, 1], [-1, 0], [0, -1]]
dire8 = [[1, 0], [1, 1], [0, 1], [-1, 1], [-1, 0], [-1, -1], [0, -1], [1, -1]]
MOD = 1000000007
INF = float("inf")

def main():
    N, K = LI()

    if N-1 <= K or comb(N-1, 2) < K:
        print(-1)
        return
    maxpairs = comb(N-1, 2)

    edges = [[] for _ in range(N)]
    edges[0] = list(range(1, N))

    pairs = list(itertools.combinations(range(1, N), 2))

    for _ in range(maxpairs - K):
        a, b = pairs.pop()
        edges[a].append(b)

    ans = []
    for a in range(N):
        for b in edges[a]:
            ans.append((a+1, b+1))

    print(len(ans))
    for a, b in ans:
        print(a, b)

if __name__ == '__main__':
    main()

Buggy submission (verdict: Runtime Error):
#!usr/bin/env python3
from collections import defaultdict, deque, Counter, OrderedDict
from bisect import bisect_left, bisect_right
from functools import reduce, lru_cache
from heapq import heappush, heappop, heapify

import itertools, bisect
import math, fractions
import sys, copy

def L(): return sys.stdin.readline().split()
def I(): return int(sys.stdin.readline().rstrip())
def SL(): return list(sys.stdin.readline().rstrip())
def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI1(): return [int(x) - 1 for x in sys.stdin.readline().split()]
def LS(): return [list(x) for x in sys.stdin.readline().split()]
def IR(n): return [I() for _ in range(n)]
def LIR(n): return [LI() for _ in range(n)]
def LIR1(n): return [LI1() for _ in range(n)]
def SR(n): return [SL() for _ in range(n)]
def LSR(n): return [LS() for _ in range(n)]
def LR(n): return [L() for _ in range(n)]

def perm(n, r): return math.factorial(n) // math.factorial(r)
def comb(n, r): return math.factorial(n) // (math.factorial(r) * math.factorial(n-r))

alphabets = "abcdefghijklmnopqrstuvwxyz"
ALPHABETS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"

sys.setrecursionlimit(1000000)
dire = [[1, 0], [0, 1], [-1, 0], [0, -1]]
dire8 = [[1, 0], [1, 1], [0, 1], [-1, 1], [-1, 0], [-1, -1], [0, -1], [1, -1]]
MOD = 1000000007
INF = float("inf")

def main():
    N, K = LI()

    maxpairs = comb(N-1, 2)
    if comb(N-1, 2) < K:
        print(-1)
        return

    edges = [[] for _ in range(N)]
    edges[0] = list(range(1, N))

    pairs = list(itertools.combinations(range(1, N), 2))

    for _ in range(maxpairs - K):
        a, b = pairs.pop()
        edges[a].append(b)

    ans = []
    for a in range(N):
        for b in edges[a]:
            ans.append((a+1, b+1))

    print(len(ans))
    for a, b in ans:
        print(a, b)

if __name__ == '__main__':
    main()