import sys, time, random
from collections import deque, Counter, defaultdict
import logging
from logging import debug, error, info, warning
logging.basicConfig(level=logging.DEBUG)
input = lambda: sys.stdin.readline().rstrip()
ii = lambda: int(input())
mi = lambda: map(int, input().split())
li = lambda: list(mi())
inf = 2 ** 61 - 1
mod = 998244353

n = ii()
a = [0] + li() + [0]

for i in range(len(a) - 1):
    if abs(a[i] - a[i + 1]) > 1:
        print('No')
        exit()
print('Yes')