Kostya likes Codeforces contests very much. However, he is very disappointed that his solutions are frequently hacked. That's why he decided to obfuscate (intentionally make less readable) his code before upcoming contest.

To obfuscate the code, Kostya first looks at the first variable name used in his program and replaces all its occurrences with a single symbol a, then he looks at the second variable name that has not been replaced yet, and replaces all its occurrences with b, and so on. Kostya is well-mannered, so he doesn't use any one-letter names before obfuscation. Moreover, there are at most 26 unique identifiers in his programs.

You are given a list of identifiers of some program with removed spaces and line breaks. Check if this program can be a result of Kostya's obfuscation.


-----Input-----

In the only line of input there is a string S of lowercase English letters (1 ≤ |S| ≤ 500) — the identifiers of a program with removed whitespace characters.


-----Output-----

If this program can be a result of Kostya's obfuscation, print "YES" (without quotes), otherwise print "NO".


-----Examples-----
Input
abacaba

Output
YES

Input
jinotega

Output
NO



-----Note-----

In the first sample case, one possible list of identifiers would be "number string number character number string number". Here how Kostya would obfuscate the program:



 replace all occurences of number with a, the result would be "a string a character a string a",

 replace all occurences of string with b, the result would be "a b a character a b a",

 replace all occurences of character with c, the result would be "a b a c a b a",

 all identifiers have been replaced, thus the obfuscation is finished.

Difficulty: competition

Starter code:

Solutions:
s = input()

last = -1
for i in range(26):
	c = chr(ord('a') + i)
	kk = s.find(c)
	if kk == -1:
		kk = len(s)
	if kk < last:
		print("NO")
		return
	last = kk
print("YES")

3

s = input()

mx = -1
for c in s:
    if ord(c) > ord('a') + mx + 1:
        print("NO")
        return
    else:
        mx = max(mx, ord(c) - ord("a"))

print("YES")


s = input()
i = 0
f = True
while len(s):
	if s[0] != chr(97 + i):
		f = False
		break
	s = s.replace(chr(97 + i), '')
	i += 1
print('YES' if f else 'NO')

import sys

next_allowed = 'a'

s = input()
for c in s:
    if ord(c) > ord(next_allowed):
        print("NO")
        return
    elif ord(c) == ord(next_allowed):
        next_allowed = chr(ord(next_allowed) + 1)

print("YES")


s = input().strip()


iter = 'a'
for i in range(len(s)):
	if ord(s[i]) < ord(iter):
		continue
	elif ord(s[i]) == ord(iter):
		iter = chr(ord(iter) + 1)
	else:
		print('NO')
		return
		
print('YES')



program = input()

to_use = "abcdefghijklmnopqrstuvwxyz"
used = ""
legal = True

for char in program:
    if char in used:
        continue
    else:
        if char == to_use[0]:
            used += to_use[0]
            to_use = to_use[1:]
        else:
            legal = False
            break
            
if legal:
    print("YES")
else:
    print("NO")

s = input()

is_correct = True

for fs in range(26):
    for sc in range(fs + 1, 26):
        is_first = s.find(chr(ord('a') + fs))
        is_second = s.find(chr(ord('a') + sc))

        if is_second != -1:
            is_correct &= is_first != -1
            is_correct &= is_first < is_second


print('YES' if is_correct else 'NO')



s = input()
w = [15234562] * 27
cnt = 0
for i in range(len(s)):
	if w[ord(s[i]) - ord('a')] == 15234562:
		w[ord(s[i]) - ord('a')] = i
		cnt += 1
for i in range(cnt):
	if w[i] >= w[i + 1]:
		print("NO")
		return
print("YES")




s = input()

mx = ord('a')
flag = True

for c in s:
    if ord(c) > mx:
        print("NO")
        flag = False
        break
    if ord(c) == mx:
        mx = mx + 1

if flag:
    print("YES")


A = "abcdefghijklmnopqrstuvwxyz"

s = input()

was = set()

s2 = ""

for c in s:
    if c not in was:
        s2 += c
        was.add(c)

print("YES" if A.startswith(s2) else "NO")

3
# Copyright (C) 2017 Sayutin Dmitry.
#
# This program is free software; you can redistribute it and/or
# modify it under the terms of the GNU General Public License as
# published by the Free Software Foundation; version 3
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; If not, see <http://www.gnu.org/licenses/>.

import sys

def main():
    has = [False] * 26
    first = [-1] * 26
    cnt = 0
    s = input().strip()
    
    for i in range(len(s)):
        num = ord(s[i]) - ord('a')
        if not has[num]:
            has[num]   = True
            first[num] = i
            cnt += 1             
    
    for i in range(cnt):
        if not has[i]:
            print("NO")
            return

    for i in range(cnt, 26):
        if has[i]:
            print("NO")
            return
    
    for i in range(1, cnt):
        if first[i - 1] > first[i]:
            print("NO")
            return
    
    print("YES")
main()




def func():
	visited = set()

	next_char = ord('a')
	for c in input():
	    if  c not in visited:
	        if ord(c) != next_char:
	            print('NO')
	            return
	        else:
	            visited.add(c)
	            next_char+=1

	print('YES')

func()

s = input()
m = ord('a') - 1
for i in range(len(s)):
    c = ord(s[i])
    if c > m + 1:
        print("NO")
        return
    m = max(m, c)
print("YES")



s = input()

current = 97

for i in s:
	if current < ord(i):
		print("NO")
		break
	elif current == ord(i):
		current += 1
else:
	print("YES")

x = input()
lastletter = 'a'
used = set()
for i in range(len(x)):
    if x[i] not in used and x[i] != lastletter:
        print("NO")
        return
    elif x[i] == lastletter:
        lastletter = chr(ord(lastletter) + 1)
    used.add(x[i])
print("YES")


s = input()
if s[0] != 'a':
    print("NO")
else:
    maxs = s[0]
    for i in range(1, len(s)):
        if ord(s[i]) - ord(maxs) > 1:
            print("NO")
            break
        if ord(s[i]) > ord(maxs):
            maxs = s[i]        
    else:
        print("YES")


s = input()
length = len(s)
a = ord('a')
i = 0
for c in s:
    if (a + i < ord(c)):
        print("NO")
        break
    if (a + i == ord(c)):
        i += 1
else:
    print("YES")

arr = list(input())
done = [False] * len(arr)

tar = list('abcdefghijklmnopqrstuvwxyz')
ti = 0

obs = True

for i in range(len(arr)):
	if done[i]:
		continue

	if tar[ti] != arr[i]:
		obs = False
		break
	
	ti += 1
	for j in range(i + 1, len(arr)):
		if done[j]:
			continue
		if arr[j] == arr[i]:
			done[j] = True

if obs:
	print('YES')
else:
	print('NO')


s = input()
l = [0] * 26
ans = True

for ch in s:
    x = ord(ch) - ord('a')
    l[x] += 1
    for i in range(0, x):
        if l[i] == 0:
            ans = False
            break
if ans:
    for i in range(0, 26):
        if l[i] == 0:
            for j in range(i + 1, 26):
                if l[j] > 0:
                    ans = False
                    break
            break

if ans:
    print('YES')
else:
    print('NO')


def main():
	s = input().strip()
	max_val = -1
	for c in s:
		n = ord(c) - ord('a')
		if n == max_val + 1:
			max_val += 1
		elif n > max_val:
			print("NO")
			return

	print("YES")

main()


def main():
    code = str(input())

    curr_a = 'a'
    while code:
        if code[0] != curr_a:
            print('NO')
            return
        code  = code.replace(curr_a, '')
        curr_a = chr(ord(curr_a) + 1)
    print('YES')
    return
main()

alpha = "abcdefghijklmnopqrstuvwxyz"
s = input()
line= []
for i in range(len(s)):
    line += [False]
yes = True
for i in range(len(alpha)):
    element = alpha[i]
    go = False
    for i in range(len(s)):
        if not line[i]:
            if s[i] == element:
                line[i] = True
                go = True
            else:
                if not go:
                    yes = False
                    break
if yes:
    print("YES")
else:
    print("NO")


def main():
    s = input()
    letters = ''
    for i in range(26):
        letters += chr(i + ord('a'))
    marked = [False] * 26
    ind = 0
    for c in s:
        if marked[ord(c) - ord('a')]:
            continue
        if letters[ind] != c:
            print('NO')
            return
        marked[ind] = True
        ind += 1
    print('YES')

main()


import sys

def debug(x, table):
    for name, val in table.items():
        if x is val:
            print('DEBUG:{} -> {}'.format(name, val), file=sys.stderr)
            return None

def solve():
    S = input()
    ch = ord('a')

    for c in S:
        if ord(c) > ch:
            print('NO')
            return
        elif ord(c) == ch:
            ch += 1
        else:
            pass
    
    print('YES')


def __starting_point():
    solve()
__starting_point()

s = input()
idents = list('abcdefghijklmnopqrstuvwxyz')
possible = True
for x in s:
    if not idents:
        break
    elif x == idents[0]:
        idents.pop(0)
    elif x not in idents:
        continue
    else:
        possible = False
print('YES' if possible else 'NO')