Monocarp has drawn a tree (an undirected connected acyclic graph) and then has given each vertex an index. All indices are distinct numbers from $1$ to $n$. For every edge $e$ of this tree, Monocarp has written two numbers: the maximum indices of the vertices of the two components formed if the edge $e$ (and only this edge) is erased from the tree.

Monocarp has given you a list of $n - 1$ pairs of numbers. He wants you to provide an example of a tree that will produce the said list if this tree exists. If such tree does not exist, say so.


-----Input-----

The first line contains one integer $n$ ($2 \le n \le 1\,000$) — the number of vertices in the tree.

Each of the next $n-1$ lines contains two integers $a_i$ and $b_i$ each ($1 \le a_i < b_i \le n$) — the maximal indices of vertices in the components formed if the $i$-th edge is removed.


-----Output-----

If there is no such tree that can produce the given list of pairs, print "NO" (without quotes).

Otherwise print "YES" (without quotes) in the first line and the edges of the tree in the next $n - 1$ lines. Each of the last $n - 1$ lines should contain two integers $x_i$ and $y_i$ ($1 \le x_i, y_i \le n$) — vertices connected by an edge.

Note: The numeration of edges doesn't matter for this task. Your solution will be considered correct if your tree produces the same pairs as given in the input file (possibly reordered). That means that you can print the edges of the tree you reconstructed in any order.


-----Examples-----
Input
4
3 4
1 4
3 4

Output
YES
1 3
3 2
2 4

Input
3
1 3
1 3

Output
NO

Input
3
1 2
2 3

Output
NO



-----Note-----

Possible tree from the first example. Dotted lines show edges you need to remove to get appropriate pairs.  [Image]

Difficulty: HARD
Tags: Greedy algorithms

Graph algorithms

Data structures

Constructive algorithms
Skill types: Data structures

Greedy algorithms

Starter code:

Solutions:
n = int(input())
V = []
for _ in range(n - 1):
	(a, b) = map(int, input().split())
	V.append(a)
	if b < n:
		print('NO')
		quit()
V.sort()
for i in range(n - 1):
	if V[i] <= i:
		print('NO')
		quit()
used = [False] * (n + 1)
tree = []
for i in range(n - 1):
	v = V[i]
	if not used[v]:
		tree.append(v)
		used[v] = True
	else:
		for j in range(1, n + 1):
			if not used[j]:
				tree.append(j)
				used[j] = True
				break
tree.append(n)
print('YES')
for i in range(n - 1):
	print(tree[i], tree[i + 1])


from sys import stdin, stdout

def three_reconstruction(n, ab_a):
	res = []
	cnt = [0] * (n + 1)
	for ab in ab_a:
		(a, b) = ab
		if a != n and b != n or a == b:
			return [False]
		cnt[min(a, b)] += 1
	cur = 0
	for i in range(1, n + 1):
		cur += cnt[i]
		if cur > i:
			return [False]
	hs = set()
	for i in range(1, n + 1):
		hs.add(i)
	last = -1
	for i in range(1, n + 1):
		if cnt[i] > 0:
			hs.remove(i)
			if last != -1:
				res.append([last, i])
			last = i
			cnt[i] -= 1
		while cnt[i] > 0:
			v = min(hs)
			res.append([last, v])
			last = v
			cnt[i] -= 1
			hs.remove(v)
	res.append([last, n])
	return [True, res]
n = int(stdin.readline())
ab_a = []
for _ in range(n - 1):
	ab_a.append(list(map(int, stdin.readline().split())))
res = three_reconstruction(n, ab_a)
if res[0]:
	stdout.write('YES\n')
	for p in res[1]:
		stdout.write(str(p[0]) + ' ' + str(p[1]) + '\n')
else:
	stdout.write('NO\n')


from copy import deepcopy
import itertools
from bisect import bisect_left
from bisect import bisect_right
import math
from collections import deque
from collections import Counter

def read():
	return int(input())

def readmap():
	return map(int, input().split())

def readlist():
	return list(map(int, input().split()))
n = read()
V = []
for _ in range(n - 1):
	(a, b) = readmap()
	V.append(a)
	if b < n:
		print('NO')
		quit()
V.sort()
for i in range(n - 1):
	if V[i] <= i:
		print('NO')
		quit()
used = [False] * (n + 1)
tree = []
for i in range(n - 1):
	v = V[i]
	if not used[v]:
		tree.append(v)
		used[v] = True
	else:
		for j in range(1, n + 1):
			if not used[j]:
				tree.append(j)
				used[j] = True
				break
tree.append(n)
print('YES')
for i in range(n - 1):
	print(tree[i], tree[i + 1])


input = __import__('sys').stdin.readline
n = int(input())
lis = []
fre = [0] * 10005
c1 = 0
ans = [[0, 0]]
has = [0] * 10005
for i in range(n - 1):
	(a, b) = map(int, input().split())
	if b < n:
		print('NO')
		exit()
	lis.append(a)
	fre[a] += 1
lis.sort()
for i in range(n - 1):
	if lis[i] <= i:
		print('NO')
		exit()
else:
	zer = 1
	for i in range(1, n):
		if fre[i] == 1:
			ans.append([i, n])
		elif fre[i] > 1:
			co = 0
			while co != fre[i] - 1:
				if fre[zer] == 0 and co == 0:
					co += 1
					ans.append([zer, n])
				elif fre[zer] == 0:
					co += 1
					ans.append([zer, ans[-1][0]])
				zer += 1
			ans.append([i, ans[-1][0]])
	print('YES')
	for i in ans[1:]:
		print(*i)


import math
import random
import heapq, bisect
import sys
from collections import deque, defaultdict
from fractions import Fraction
import sys
from collections import defaultdict
mod = 10 ** 9 + 7
mod1 = 998244353
import os
import sys
from io import BytesIO, IOBase
BUFSIZE = 8192

class FastIO(IOBase):
	newlines = 0

	def __init__(self, file):
		self._fd = file.fileno()
		self.buffer = BytesIO()
		self.writable = 'x' in file.mode or 'r' not in file.mode
		self.write = self.buffer.write if self.writable else None

	def read(self):
		while True:
			b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
			if not b:
				break
			ptr = self.buffer.tell()
			(self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr))
		self.newlines = 0
		return self.buffer.read()

	def readline(self):
		while self.newlines == 0:
			b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
			self.newlines = b.count(b'\n') + (not b)
			ptr = self.buffer.tell()
			(self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr))
		self.newlines -= 1
		return self.buffer.readline()

	def flush(self):
		if self.writable:
			os.write(self._fd, self.buffer.getvalue())
			(self.buffer.truncate(0), self.buffer.seek(0))

class IOWrapper(IOBase):

	def __init__(self, file):
		self.buffer = FastIO(file)
		self.flush = self.buffer.flush
		self.writable = self.buffer.writable
		self.write = lambda s: self.buffer.write(s.encode('ascii'))
		self.read = lambda : self.buffer.read().decode('ascii')
		self.readline = lambda : self.buffer.readline().decode('ascii')
(sys.stdin, sys.stdout) = (IOWrapper(sys.stdin), IOWrapper(sys.stdout))
input = lambda : sys.stdin.readline().rstrip('\r\n')

class TreeNode:

	def __init__(self, k, v):
		self.key = k
		self.value = v
		self.left = None
		self.right = None
		self.parent = None
		self.height = 1
		self.num_left = 1
		self.num_total = 1

class AvlTree:

	def __init__(self):
		self._tree = None

	def add(self, k, v):
		if not self._tree:
			self._tree = TreeNode(k, v)
			return
		node = self._add(k, v)
		if node:
			self._rebalance(node)

	def _add(self, k, v):
		node = self._tree
		while node:
			if k < node.key:
				if node.left:
					node = node.left
				else:
					node.left = TreeNode(k, v)
					node.left.parent = node
					return node.left
			elif node.key < k:
				if node.right:
					node = node.right
				else:
					node.right = TreeNode(k, v)
					node.right.parent = node
					return node.right
			else:
				node.value = v
				return

	@staticmethod
	def get_height(x):
		return x.height if x else 0

	@staticmethod
	def get_num_total(x):
		return x.num_total if x else 0

	def _rebalance(self, node):
		n = node
		while n:
			lh = self.get_height(n.left)
			rh = self.get_height(n.right)
			n.height = max(lh, rh) + 1
			balance_factor = lh - rh
			n.num_total = 1 + self.get_num_total(n.left) + self.get_num_total(n.right)
			n.num_left = 1 + self.get_num_total(n.left)
			if balance_factor > 1:
				if self.get_height(n.left.left) < self.get_height(n.left.right):
					self._rotate_left(n.left)
				self._rotate_right(n)
			elif balance_factor < -1:
				if self.get_height(n.right.right) < self.get_height(n.right.left):
					self._rotate_right(n.right)
				self._rotate_left(n)
			else:
				n = n.parent

	def _remove_one(self, node):
		replacement = node.left or node.right
		if node.parent:
			if AvlTree._is_left(node):
				node.parent.left = replacement
			else:
				node.parent.right = replacement
			replacement.parent = node.parent
			node.parent = None
		else:
			self._tree = replacement
			replacement.parent = None
		node.left = None
		node.right = None
		node.parent = None
		self._rebalance(replacement)

	def _remove_leaf(self, node):
		if node.parent:
			if AvlTree._is_left(node):
				node.parent.left = None
			else:
				node.parent.right = None
			self._rebalance(node.parent)
		else:
			self._tree = None
		node.parent = None
		node.left = None
		node.right = None

	def remove(self, k):
		node = self._get_node(k)
		if not node:
			return
		if AvlTree._is_leaf(node):
			self._remove_leaf(node)
			return
		if node.left and node.right:
			nxt = AvlTree._get_next(node)
			node.key = nxt.key
			node.value = nxt.value
			if self._is_leaf(nxt):
				self._remove_leaf(nxt)
			else:
				self._remove_one(nxt)
			self._rebalance(node)
		else:
			self._remove_one(node)

	def get(self, k):
		node = self._get_node(k)
		return node.value if node else -1

	def _get_node(self, k):
		if not self._tree:
			return None
		node = self._tree
		while node:
			if k < node.key:
				node = node.left
			elif node.key < k:
				node = node.right
			else:
				return node
		return None

	def get_at(self, pos):
		x = pos + 1
		node = self._tree
		while node:
			if x < node.num_left:
				node = node.left
			elif node.num_left < x:
				x -= node.num_left
				node = node.right
			else:
				return (node.key, node.value)
		raise IndexError('Out of ranges')

	@staticmethod
	def _is_left(node):
		return node.parent.left and node.parent.left == node

	@staticmethod
	def _is_leaf(node):
		return node.left is None and node.right is None

	def _rotate_right(self, node):
		if not node.parent:
			self._tree = node.left
			node.left.parent = None
		elif AvlTree._is_left(node):
			node.parent.left = node.left
			node.left.parent = node.parent
		else:
			node.parent.right = node.left
			node.left.parent = node.parent
		bk = node.left.right
		node.left.right = node
		node.parent = node.left
		node.left = bk
		if bk:
			bk.parent = node
		node.height = max(self.get_height(node.left), self.get_height(node.right)) + 1
		node.num_total = 1 + self.get_num_total(node.left) + self.get_num_total(node.right)
		node.num_left = 1 + self.get_num_total(node.left)

	def _rotate_left(self, node):
		if not node.parent:
			self._tree = node.right
			node.right.parent = None
		elif AvlTree._is_left(node):
			node.parent.left = node.right
			node.right.parent = node.parent
		else:
			node.parent.right = node.right
			node.right.parent = node.parent
		bk = node.right.left
		node.right.left = node
		node.parent = node.right
		node.right = bk
		if bk:
			bk.parent = node
		node.height = max(self.get_height(node.left), self.get_height(node.right)) + 1
		node.num_total = 1 + self.get_num_total(node.left) + self.get_num_total(node.right)
		node.num_left = 1 + self.get_num_total(node.left)

	@staticmethod
	def _get_next(node):
		if not node.right:
			return node.parent
		n = node.right
		while n.left:
			n = n.left
		return n

class SegmentTree1:

	def __init__(self, data, default=300006, func=lambda a, b: min(a, b)):
		self._default = default
		self._func = func
		self._len = len(data)
		self._size = _size = 1 << (self._len - 1).bit_length()
		self.data = [default] * (2 * _size)
		self.data[_size:_size + self._len] = data
		for i in reversed(range(_size)):
			self.data[i] = func(self.data[i + i], self.data[i + i + 1])

	def __delitem__(self, idx):
		self[idx] = self._default

	def __getitem__(self, idx):
		return self.data[idx + self._size]

	def __setitem__(self, idx, value):
		idx += self._size
		self.data[idx] = value
		idx >>= 1
		while idx:
			self.data[idx] = self._func(self.data[2 * idx], self.data[2 * idx + 1])
			idx >>= 1

	def __len__(self):
		return self._len

	def query(self, start, stop):
		if start == stop:
			return self.__getitem__(start)
		stop += 1
		start += self._size
		stop += self._size
		res = self._default
		while start < stop:
			if start & 1:
				res = self._func(res, self.data[start])
				start += 1
			if stop & 1:
				stop -= 1
				res = self._func(res, self.data[stop])
			start >>= 1
			stop >>= 1
		return res

	def __repr__(self):
		return 'SegmentTree({0})'.format(self.data)

class SegmentTree:

	def __init__(self, data, default=0, func=lambda a, b: a + b):
		self._default = default
		self._func = func
		self._len = len(data)
		self._size = _size = 1 << (self._len - 1).bit_length()
		self.data = [default] * (2 * _size)
		self.data[_size:_size + self._len] = data
		for i in reversed(range(_size)):
			self.data[i] = func(self.data[i + i], self.data[i + i + 1])

	def __delitem__(self, idx):
		self[idx] = self._default

	def __getitem__(self, idx):
		return self.data[idx + self._size]

	def __setitem__(self, idx, value):
		idx += self._size
		self.data[idx] = value
		idx >>= 1
		while idx:
			self.data[idx] = self._func(self.data[2 * idx], self.data[2 * idx + 1])
			idx >>= 1

	def __len__(self):
		return self._len

	def query(self, start, stop):
		if start == stop:
			return self.__getitem__(start)
		stop += 1
		start += self._size
		stop += self._size
		res = self._default
		while start < stop:
			if start & 1:
				res = self._func(res, self.data[start])
				start += 1
			if stop & 1:
				stop -= 1
				res = self._func(res, self.data[stop])
			start >>= 1
			stop >>= 1
		return res

	def __repr__(self):
		return 'SegmentTree({0})'.format(self.data)

class Factorial:

	def __init__(self, MOD):
		self.MOD = MOD
		self.factorials = [1, 1]
		self.invModulos = [0, 1]
		self.invFactorial_ = [1, 1]

	def calc(self, n):
		if n <= -1:
			print('Invalid argument to calculate n!')
			print('n must be non-negative value. But the argument was ' + str(n))
			exit()
		if n < len(self.factorials):
			return self.factorials[n]
		nextArr = [0] * (n + 1 - len(self.factorials))
		initialI = len(self.factorials)
		prev = self.factorials[-1]
		m = self.MOD
		for i in range(initialI, n + 1):
			prev = nextArr[i - initialI] = prev * i % m
		self.factorials += nextArr
		return self.factorials[n]

	def inv(self, n):
		if n <= -1:
			print('Invalid argument to calculate n^(-1)')
			print('n must be non-negative value. But the argument was ' + str(n))
			exit()
		p = self.MOD
		pi = n % p
		if pi < len(self.invModulos):
			return self.invModulos[pi]
		nextArr = [0] * (n + 1 - len(self.invModulos))
		initialI = len(self.invModulos)
		for i in range(initialI, min(p, n + 1)):
			next = -self.invModulos[p % i] * (p // i) % p
			self.invModulos.append(next)
		return self.invModulos[pi]

	def invFactorial(self, n):
		if n <= -1:
			print('Invalid argument to calculate (n^(-1))!')
			print('n must be non-negative value. But the argument was ' + str(n))
			exit()
		if n < len(self.invFactorial_):
			return self.invFactorial_[n]
		self.inv(n)
		nextArr = [0] * (n + 1 - len(self.invFactorial_))
		initialI = len(self.invFactorial_)
		prev = self.invFactorial_[-1]
		p = self.MOD
		for i in range(initialI, n + 1):
			prev = nextArr[i - initialI] = prev * self.invModulos[i % p] % p
		self.invFactorial_ += nextArr
		return self.invFactorial_[n]

class Combination:

	def __init__(self, MOD):
		self.MOD = MOD
		self.factorial = Factorial(MOD)

	def ncr(self, n, k):
		if k < 0 or n < k:
			return 0
		k = min(k, n - k)
		f = self.factorial
		return f.calc(n) * f.invFactorial(max(n - k, k)) * f.invFactorial(min(k, n - k)) % self.MOD

def powm(a, n, m):
	if a == 1 or n == 0:
		return 1
	if n % 2 == 0:
		s = powm(a, n // 2, m)
		return s * s % m
	else:
		return a * powm(a, n - 1, m) % m

def sort_list(list1, list2):
	zipped_pairs = zip(list2, list1)
	z = [x for (_, x) in sorted(zipped_pairs)]
	return z

def product(l):
	por = 1
	for i in range(len(l)):
		por *= l[i]
	return por

def binarySearchCount(arr, n, key):
	left = 0
	right = n - 1
	count = 0
	while left <= right:
		mid = int((right + left) / 2)
		if arr[mid] <= key:
			count = mid + 1
			left = mid + 1
		else:
			right = mid - 1
	return count

def countdig(n):
	c = 0
	while n > 0:
		n //= 10
		c += 1
	return c

def binary(x, length):
	y = bin(x)[2:]
	return y if len(y) >= length else '0' * (length - len(y)) + y

def countGreater(arr, n, k):
	l = 0
	r = n - 1
	leftGreater = n
	while l <= r:
		m = int(l + (r - l) / 2)
		if arr[m] >= k:
			leftGreater = m
			r = m - 1
		else:
			l = m + 1
	return n - leftGreater

class TrieNode:

	def __init__(self):
		self.children = [None] * 26
		self.isEndOfWord = False

class Trie:

	def __init__(self):
		self.root = self.getNode()

	def getNode(self):
		return TrieNode()

	def _charToIndex(self, ch):
		return ord(ch) - ord('a')

	def insert(self, key):
		pCrawl = self.root
		length = len(key)
		for level in range(length):
			index = self._charToIndex(key[level])
			if not pCrawl.children[index]:
				pCrawl.children[index] = self.getNode()
			pCrawl = pCrawl.children[index]
		pCrawl.isEndOfWord = True

	def search(self, key):
		pCrawl = self.root
		length = len(key)
		for level in range(length):
			index = self._charToIndex(key[level])
			if not pCrawl.children[index]:
				return False
			pCrawl = pCrawl.children[index]
		return pCrawl != None and pCrawl.isEndOfWord

class Node:

	def __init__(self, data):
		self.data = data
		self.count = 0
		self.left = None
		self.right = None

class BinaryTrie:

	def __init__(self):
		self.root = Node(0)

	def insert(self, pre_xor):
		self.temp = self.root
		for i in range(31, -1, -1):
			val = pre_xor & 1 << i
			if val:
				if not self.temp.right:
					self.temp.right = Node(0)
				self.temp = self.temp.right
				self.temp.count += 1
			if not val:
				if not self.temp.left:
					self.temp.left = Node(0)
				self.temp = self.temp.left
				self.temp.count += 1
		self.temp.data = pre_xor

	def query(self, xor):
		self.temp = self.root
		for i in range(31, -1, -1):
			val = xor & 1 << i
			if not val:
				if self.temp.left and self.temp.left.count > 0:
					self.temp = self.temp.left
				elif self.temp.right:
					self.temp = self.temp.right
			elif self.temp.right and self.temp.right.count > 0:
				self.temp = self.temp.right
			elif self.temp.left:
				self.temp = self.temp.left
			self.temp.count -= 1
		return xor ^ self.temp.data
n = int(input())
ans = []
f = 0
d = defaultdict(int)
for i in range(n - 1):
	(a, b) = map(int, input().split())
	if b != n:
		f = 1
	d[a] += 1
t = n - 1
if f == 1:
	print('NO')
	sys.exit(0)
for i in sorted(d, reverse=True):
	last = i
	t = min(t, i - 1)
	while t in d:
		t -= 1
	for j in range(d[i] - 1):
		if t <= 0:
			print('NO')
			sys.exit(0)
		ans.append((last, t))
		last = t
		t -= 1
		while t in d:
			t -= 1
	ans.append((n, last))
print('YES')
for i in range(n - 1):
	print(*ans[i])


from bisect import bisect
from collections import defaultdict
from math import gcd, sqrt, ceil
from collections import Counter
import sys
sys.setrecursionlimit(10 ** 9)
ans = []
n = int(input())
ba = [i for i in range(1, n + 1)]
yo = set((i for i in range(1, n + 1)))
la = []
hash = defaultdict(int)
for i in range(n - 1):
	(a, b) = map(int, input().split())
	la.append([max(a, b), min(a, b)])
	if a != n and b != n:
		print('NO')
		exit()
	if a == b:
		print('NO')
		exit()
	hash[min(a, b)] += 1
seti = set()
for i in range(1, n):
	z = hash[i]
	j = i - 2
	if z == 0:
		continue
	if z == 1:
		ans.append([i, n])
		yo.remove(i)
		continue
	z -= 1
	if z > j:
		print('NO')
		exit()
	if j < 0:
		print('NO')
		exit()
	now = i
	ha = []
	count = 0
	while j >= 0:
		if count == z:
			break
		if ba[j] in yo:
			ans.append([now, ba[j]])
			yo.remove(now)
			now = ba[j]
			j -= 1
			count += 1
		else:
			j -= 1
	if count != z:
		print('NO')
		exit()
	yo.remove(ans[-1][1])
	ans.append([ans[-1][1], n])
print('YES')
for (a, b) in ans:
	print(a, b)


n = int(input())
a = []
fr = [0] * 10000
for i in range(n - 1):
	(x, y) = map(int, input().split())
	if x == y or (x < n and y < n):
		print('NO')
		exit(0)
	if x == n:
		a.append(y)
	else:
		a.append(x)
b = []
for i in range(1, n + 1):
	if i not in a:
		b.append(i)
for i in a:
	if fr[i] == 0:
		fr[i] = n
		continue
	tep = 0
	record = 0
	for j in range(len(b)):
		if b[j] and b[j] < i:
			if b[j] > tep:
				tep = b[j]
				record = j
	b[record] = 0
	if tep == 0:
		print('NO')
		exit(0)
	fr[tep] = fr[i]
	fr[i] = tep
print('YES')
for i in range(1, n):
	print(i, fr[i])


import sys
n = int(input())
mentioned = [0] * (n - 1)
for _ in range(n - 1):
	(a, b) = map(int, input().split())
	if b != n:
		print('NO')
		sys.exit()
	else:
		mentioned[a - 1] += 1
		if mentioned[a - 1] > a:
			print('NO')
			sys.exit()
if mentioned[-1] == 0:
	print('NO')
	sys.exit()
cnt = 0
for i in range(n - 2, -1, -1):
	cnt += mentioned[i] - 1
	if cnt < 0:
		print('NO')
		sys.exit()
print('YES')
end = [i for i in range(n - 1) if mentioned[i] > 0]
mid = [i for i in range(n - 1) if mentioned[i] == 0]
branch_sum = [mentioned[end[0]] - 1]
for i in range(1, len(end)):
	branch_sum.append(branch_sum[-1] + mentioned[end[i]] - 1)
branch = [[] for _ in range(len(end))]
branch[0] = mid[:branch_sum[0]]
for i in range(1, len(end)):
	branch[i] = mid[branch_sum[i - 1]:branch_sum[i]]
for i in range(len(end)):
	ans = [end[i] + 1]
	ans += [j + 1 for j in branch[i]]
	ans.append(n)
	for j in range(len(ans) - 1):
		print(ans[j], ans[j + 1])


n = int(input())
indices = []
for i in range(n - 1):
	(x, y) = map(int, input().split())
	indices.append((x, y))
try:
	not_seen = set(list(range(1, n)))
	seen = {}
	for (x, y) in indices:
		assert x == n or y == n
		seen.setdefault(min(x, y), 0)
		seen[min(x, y)] += 1
		if min(x, y) in not_seen:
			not_seen.remove(min(x, y))
	not_seen = sorted(list(not_seen), reverse=True)
	seen_list = list(seen.keys())
	seen_list.sort(reverse=True)
	edges = []
	while seen_list:
		cur_elem = seen_list.pop(0)
		prev = n
		while seen[cur_elem] != 1:
			assert not_seen[0] < cur_elem
			next_elem = not_seen.pop(0)
			edges.append((prev, next_elem))
			prev = next_elem
			seen[cur_elem] -= 1
		edges.append((prev, cur_elem))
	print('YES')
	for (x, y) in edges:
		print(x, y)
except AssertionError:
	print('NO')


from sys import stdin

def bad():
	print('NO')
	exit()
all_in = stdin.readlines()
n = int(all_in[0])
pair = list(map(lambda x: tuple(map(int, x.split())), all_in[1:]))
f = True
for i in range(n - 1):
	if pair[i][0] == n - 1:
		f = False
	if pair[i][1] != n:
		bad()
if f:
	bad()
pair.sort(key=lambda x: x[0])
p = list(map(lambda x: x[0], pair))
ans = [0 for i in range(n)]
st = set(range(1, n))
ans[0] = p[0]
st.remove(p[0])
(a, b) = (0, p[0])
for i in range(1, n - 1):
	(a, b) = (b, p[i])
	if b != a:
		ans[i] = b
		st.remove(b)
ans[-1] = n
max_ = 0
for i in range(n):
	el = ans[i]
	max_ = max(max_, el)
	if not el:
		m = min(st)
		if m > max_:
			bad()
		ans[i] = m
		st.remove(m)
print('YES')
print('\n'.join(map(lambda i: f'{ans[i - 1]} {ans[i]}', range(1, n))))


def solve():
	n = int(input())
	d = {}
	for _ in range(n - 1):
		(u, v) = map(int, input().split())
		min_ = min(u, v)
		max_ = max(u, v)
		if max_ != n:
			return (False, None)
		if min_ not in d:
			d[min_] = 0
		d[min_] += 1
	if sum(list(d.values())) + 1 != n:
		return (False, None)
	edge = []
	used = {i: False for i in range(1, n + 1)}
	for k in sorted(list(d.keys())):
		used[k] = True
		mid = [n]
		for i in range(k - 1, 0, -1):
			if len(mid) == d[k]:
				break
			if used[i] == False:
				used[i] = True
				mid.append(i)
		if len(mid) < d[k]:
			return (False, None)
		mid.append(k)
		for (u, v) in zip(mid[:-1], mid[1:]):
			edge.append([u, v])
	return (True, edge)
(ans, arr) = solve()
if ans == False:
	print('NO')
else:
	print('YES')
	for (u, v) in arr:
		print(str(u) + ' ' + str(v))