One particularly well-known fact about zombies is that they move and think terribly slowly. While we still don't know why their movements are so sluggish, the problem of laggy thinking has been recently resolved. It turns out that the reason is not (as previously suspected) any kind of brain defect – it's the opposite! Independent researchers confirmed that the nervous system of a zombie is highly complicated – it consists of n brains (much like a cow has several stomachs). They are interconnected by brain connectors, which are veins capable of transmitting thoughts between brains. There are two important properties such a brain network should have to function properly:   It should be possible to exchange thoughts between any two pairs of brains (perhaps indirectly, through other brains).  There should be no redundant brain connectors, that is, removing any brain connector would make property 1 false. 

If both properties are satisfied, we say that the nervous system is valid. Unfortunately (?), if the system is not valid, the zombie stops thinking and becomes (even more) dead. Your task is to analyze a given nervous system of a zombie and find out whether it is valid.


-----Input-----

The first line of the input contains two space-separated integers n and m (1 ≤ n, m ≤ 1000) denoting the number of brains (which are conveniently numbered from 1 to n) and the number of brain connectors in the nervous system, respectively. In the next m lines, descriptions of brain connectors follow. Every connector is given as a pair of brains a b it connects (1 ≤ a, b ≤ n, a ≠ b).


-----Output-----

The output consists of one line, containing either yes or no depending on whether the nervous system is valid.


-----Examples-----
Input
4 4
1 2
2 3
3 1
4 1

Output
no

Input
6 5
1 2
2 3
3 4
4 5
3 6

Output
yes

Source: codeforces | Difficulty: EASY


Solutions:
f = lambda : map(int, input().split())
(n, m) = f()
p = list(range(n))

def g(i):
	while i != p[i]:
		i = p[i]
	return i
q = 'yes' if m == n - 1 else 'no'
for j in range(m):
	(a, b) = f()
	(u, v) = (g(a - 1), g(b - 1))
	if u == v:
		q = 'no'
	p[u] = v
print(q)


read = lambda : map(int, input().split())
(n, m) = read()
g = [list() for i in range(n + 1)]
for i in range(m):
	(a, b) = read()
	g[a].append(b)
	g[b].append(a)

def dfs(v):
	was[v] = 1
	for u in g[v]:
		if not was[u]:
			dfs(u)
was = [0] * (n + 1)
dfs(1)
ans = 'no' if 0 in was[1:] or m != n - 1 else 'yes'
print(ans)


(n, m) = list(map(int, input().split()))
ok = m == n - 1
dsu = list(range(n))

def get(x):
	y = x
	while y != dsu[y]:
		y = dsu[y]
	while x != y:
		z = dsu[x]
		dsu[x] = y
		x = z
	return y
for i in range(m):
	(a, b) = [int(x) - 1 for x in input().split()]
	dsu[get(a)] = get(b)
for i in range(n):
	get(i)
ok &= max(dsu) == min(dsu)
print('yes' if ok else 'no')


def clave(ent):
	return min(ent)
a = [int(i) for i in input().split()]
t = set((int(i) for i in input().split()))
x = []
for i in range(1, a[1]):
	x.append([int(i) for i in input().split()])
x.sort(key=clave)
while len(x):
	f = True
	for c in x:
		if (c[0] in t) + (c[1] in t) == 1:
			f = False
			t.update(c)
			x.remove(c)
			break
	if f:
		print('no')
		return
print('yes')


(n, m) = map(int, input().split())
used = []
graph = []
for i in range(n + 10):
	used.append(0)
	graph.append([])
for _ in range(m):
	(u, v) = map(int, input().split())
	graph[u].append(v)
	graph[v].append(u)

def dfs(v, pr):
	used[v] = 1
	for to in graph[v]:
		if used[to] == 1 and pr != to:
			print('no')
			exit()
		if used[to] != 1:
			dfs(to, v)
components = 0
for i in range(1, n + 1):
	if not used[i]:
		components += 1
		dfs(i, -1)
print('yes' if components == 1 else 'no')


def adds(sin, c):
	if c in sin:
		return 1
	sin.add(c)
	return 0

def corr(v, n):
	if len(v) < n - 1:
		return 0
	sin = {v[0][0], v[0][1]}
	v = v[1:]
	while len(v) > 0:
		nv = []
		for c in v:
			if c[0] in sin:
				if adds(sin, c[1]):
					return 0
				continue
			if c[1] in sin:
				if adds(sin, c[0]):
					return 0
				continue
			nv.append(c)
		if v == nv:
			return 0
		v = nv
	if len(sin) == n:
		return 1
	else:
		return 0
(n, m) = map(int, input().split(' '))
v = []
for c in range(m):
	v.append(list(map(int, input().split(' '))))
if corr(v, n):
	print('yes')
else:
	print('no')


k = [0] * 1001

def dfs(x):
	k[x] += 1
	for i in z[x]:
		if not k[i]:
			dfs(i)
(a, b) = list(map(int, input().split()))
z = []
for i in range(a + 1):
	z += [[]]
for i in range(b):
	(x, y) = list(map(int, input().split()))
	z[x].append(y)
	z[y].append(x)
dfs(1)
if sum(k) != a or a - 1 != b:
	print('no')
else:
	print('yes')


l = []
were = []

def dfs(v):
	global l, were
	if not were:
		were = len(l) * [False]
	were[v] = True
	for i in l[v]:
		if not were[i]:
			dfs(i)
(n, m) = map(int, input().split())
if m != n - 1:
	print('no')
else:
	l = [[] for i in range(n)]
	for i in range(m):
		(a, b) = map(int, input().split())
		l[a - 1].append(b - 1)
		l[b - 1].append(a - 1)
	dfs(0)
	f = True
	for i in were:
		f = f and i
	if f:
		print('yes')
	else:
		print('no')


import sys
sys.setrecursionlimit(1000000)

def dfs(v, pr):
	nonlocal used
	nonlocal p
	nonlocal f
	if not f:
		return None
	if used[v]:
		f = False
	used[v] = True
	for i in range(len(p[v])):
		if p[v][i] != pr:
			dfs(p[v][i], v)
(n, m) = list(map(int, input().split()))
p = []
for i in range(n):
	p.append([])
for i in range(m):
	(a, b) = list(map(int, input().split()))
	a -= 1
	b -= 1
	p[a].append(b)
	p[b].append(a)
used = [False] * n
f = True
for i in range(n):
	if i != 0 and (not used[i]):
		f = False
		break
	if not used[i]:
		dfs(i, -1)
if f:
	print('yes')
else:
	print('no')


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')
from types import GeneratorType

def iterative(f, stack=[]):

	def wrapped_func(*args, **kwargs):
		if stack:
			return f(*args, **kwargs)
		to = f(*args, **kwargs)
		while True:
			if type(to) is GeneratorType:
				stack.append(to)
				to = next(to)
				continue
			stack.pop()
			if not stack:
				break
			to = stack[-1].send(to)
		return to
	return wrapped_func
node = 0
ans = 0

def dfs(s):
	v[s] = 1
	for i in g[s]:
		if v[i] == 0:
			v[i] = 1
			dfs(i)
(n, k) = map(int, input().split())
g = [[] for i in range(n + 1)]
for i in range(k):
	(u, v) = map(int, input().split())
	g[u].append(v)
	g[v].append(u)
v = [0] * (n + 1)
dfs(1)
if v[1:] == [1] * n:
	print('yes' if k == n - 1 else 'no')
else:
	print('no')


from sys import stdin as fin

def dfs(v, p):
	global g, used, cnt
	cnt += 1
	used[v] = True
	for u in g[v]:
		if u != p and (used[u] or not dfs(u, v)):
			return False
	return True
(n, m) = map(int, fin.readline().split())
used = [False] * n
g = [[] for i in range(n)]
for i in range(m):
	(v, u) = map(int, fin.readline().split())
	(v, u) = (v - 1, u - 1)
	g[v].append(u)
	g[u].append(v)
cnt = 0
if not dfs(0, None) or cnt != n:
	print('no')
else:
	print('yes')


def is_valid():
	import sys
	with sys.stdin as f:
		for (i, line) in enumerate(f):
			if i == 0:
				(N, M) = line.split(' ')
				(N, M) = (int(N), int(M))
				if N - 1 != M:
					return False
				graph = [[] for _ in range(N)]
			else:
				(fromVertex, toVertex) = line.split(' ')
				(fromVertex, toVertex) = (int(fromVertex) - 1, int(toVertex) - 1)
				graph[fromVertex].append(toVertex)
				graph[toVertex].append(fromVertex)
	from queue import LifoQueue

	def is_single_component(start_node, graph):
		N = len(graph)
		visited = [False for _ in range(N)]
		visited[start_node] = True
		nodes_queue = LifoQueue()
		nodes_queue.put(start_node)
		while not nodes_queue.empty():
			node = nodes_queue.get()
			for neigh in graph[node]:
				if not visited[neigh]:
					visited[neigh] = True
					nodes_queue.put(neigh)
		return all(visited)
	return is_single_component(0, graph)
if is_valid():
	print('yes')
else:
	print('no')


def dfs(start, vis, end, ans):
	global graph
	vis[start] = True
	if start == end:
		ans[0] = True
		return True
	for node in graph[start]:
		if node not in vis:
			dfs(node, vis, end, ans)
from collections import defaultdict
(n, m) = map(int, input().split())
graph = defaultdict(list)
flag = True
arr = []
for i in range(m):
	(u, v) = map(int, input().split())
	arr.append([u, v])
for i in range(m):
	(u, v) = arr[i]
	if u in graph and v in graph:
		vis = {}
		ans = [False]
		dfs(u, vis, v, ans)
		if ans[0]:
			flag = False
			break
	graph[u].append(v)
	graph[v].append(u)
if n - m > 1:
	flag = False
if not flag:
	print('no')
else:
	print('yes')


import sys
from collections import defaultdict

class Graph:

	def __init__(self, V):
		self.V = V
		self.graph = defaultdict(list)

	def addEdge(self, v, w):
		self.graph[v].append(w)
		self.graph[w].append(v)

	def isCyclicUtil(self, v, visited, parent):
		visited[v] = True
		for i in self.graph[v]:
			if visited[i] == False:
				if self.isCyclicUtil(i, visited, v) == True:
					return True
			elif i != parent:
				return True
		return False

	def isTree(self):
		visited = [False] * self.V
		if self.isCyclicUtil(0, visited, -1) == True:
			return False
		for i in range(self.V):
			if visited[i] == False:
				return False
		return True

def main():
	s = sys.stdin.readline().split()
	(n, m) = (int(s[0]), int(s[1]))
	graph = Graph(n)
	for _ in range(m):
		s = sys.stdin.readline().split()
		graph.addEdge(int(s[0]) - 1, int(s[1]) - 1)
	return 'yes' if graph.isTree() else 'no'
sys.stdout.write(main())


3
__import__('sys').setrecursionlimit(10 ** 6)

def dfs(u, tr, used):
	used[u] = True
	ans = 1
	for v in tr[u]:
		if not used[v]:
			ans += dfs(v, tr, used)
	return ans
(n, m) = list(map(int, input().split()))
if n != m + 1:
	print('no')
	return
tr = [[] for i in range(n)]
for i in range(m):
	(a, b) = list(map(int, input().split()))
	a -= 1
	b -= 1
	tr[a].append(b)
	tr[b].append(a)
used = [False] * n
if dfs(0, tr, used) == n:
	print('yes')
else:
	print('no')


f = lambda : map(int, input().split())
(n, m) = f()
p = list(range(n))

def g(i):
	j = i
	while j != p[j]:
		j = p[j]
	while i != j:
		(i, p[i]) = (p[i], j)
	return j
for j in range(m):
	(a, b) = f()
	p[g(a - 1)] = g(b - 1)
s = g(0)
k = all((g(i) == s for i in p))
print('yes' if m == n - 1 and k else 'no')