4
90 91 90 91
 4
54 54 54 55
 10
55 39 93 42 97 40 36 38 11 97
 3
90 39 98
 5
21 57 40 94 17
 8
46 4 30 85 52 6 84 13
 6
47 100 96 2 96 43
 4
61 28 3 81
 5
14 91 91 91 84
 3
22 23 24
 1
13
 9
13 80 13 38 98 85 11 73 74
 10
95 51 52 8 44 39 77 17 96 88
 3
40 40 40
 4
16 17 17 18
 3
68 67 67
 4
83 100 81 75
 9
71 71 70 70 71 70 69 70 71
 10
9 9 9 10 10 9 8 8 9 9
 6
87 86 86 86 85 86
 5
64 63 63 62 61
 3
76 9 12
 4
61 60 60 60
 7
23 37 39 8 72 31 85
 10
67 67 67 67 67 67 67 67 67 67
 4
92 18 29 93
 4
14 15 15 16
 10
79 84 22 38 23 22 33 42 13 96
 7
20 21 95 73 49 98 53
 10
28 61 23 73 61 33 45 55 18 43
 9
15 14 14 13 13 12 13 13 14
 5
46 46 47 46 45
 7
35 91 87 78 17 71 63
 3
69 26 73
 10
16 17 16 15 14 15 16 17 16 15
 9
95 7 97 61 90 7 30 65 39
 10
2 83 18 65 58 95 37 51 86 47
 6
59 59 60 60 59 58

YES YES NO NO NO NO NO NO NO YES YES NO NO YES YES YES NO YES YES YES YES NO YES NO YES NO YES NO NO NO YES YES NO NO YES NO NO YES
"""
Codeforces April Fools 2018 Problem C

Author  : chaotic_iak
Language: Python 3.5.2
"""

################################################### SOLUTION

def initialize_solution():
    pass

def main():
    n, = read()
    a = read()
    for i in range(n-1):
        if abs(a[i] - a[i+1]) >= 2:
            return "NO"
    return "YES"

########################################## PROBLEM CONSTANTS

READ_FROM_FILE = None
OUTPUT_PREFIX = None
INTERACTIVE = False

#################################################### HELPERS

import sys

def read(callback=int, split=True):
    if READ_FROM_FILE:
        ipt = sfile.readline().strip()
    else:
        ipt = input().strip()
    if INTERACTIVE and ipt == "WRONG_ANSWER":
        sys.exit()
    if split:
        return list(map(callback, ipt.split()))
    else:
        return callback(ipt)

def write(value, end="\n"):
    if value is None: return
    try:
        if not isinstance(value, str):
            value = " ".join(map(str, value))
    except:
        pass
    if READ_FROM_FILE:
        tfile.write(str(value, end=end))
    else:
        print(value, end=end)
    if INTERACTIVE:
        sys.stdout.flush()

sfile = None
tfile = None
if READ_FROM_FILE:
    sfile = open(READ_FROM_FILE + ".in", "r")
    sfile.seek(0)
    tfile = open(READ_FROM_FILE + ".out", "w")
if OUTPUT_PREFIX is None:
    result = main()
    if result is not None:
        write(result)
else:
    initialize_solution()
    TOTAL_CASES, = read()
    for CASE_NUMBER in range(1, TOTAL_CASES+1):
        write(OUTPUT_PREFIX.replace("%d", str(CASE_NUMBER)), end="")
        result = main()
        if result is not None:
            write(result)
if sfile is not None: sfile.close()
if tfile is not None: tfile.close()
