n = int(input())
a = list(map(int,input().split()))
if n == 1:
    if a[0] in [0,1]:
        print('Yes')
    else:
        print('No')
    exit()
if min(a) >= 2:
    exit(print('No'))
x = []
temp = [a[0]]
for i in range(1,n):
    if abs(temp[-1] - a[i]) <= 1:
        temp.append(a[i])
    else:
        x.append(temp)
        temp = [a[i]]
if temp:
    x.append(temp)

ans = 'Yes'
for i in x:
    z = max(i)
    y = i.index(z)
    s = set()
    for j in range(y):
        s.add(i[j+1] - i[j])
    if len(s) >= 3 or (1 in s and -1 in s):
        ans = 'No'
    s.discard(0)
    s.discard(-1)
    s.discard(1)
    if len(s) > 0:
        ans = 'No'
    s = set()
    for j in range(y,len(i)-1):
        s.add(i[j+1] - i[j])
    if len(s) >= 3 or (1 in s and -1 in s):
        ans = 'No'
    s.discard(0)
    s.discard(-1)
    s.discard(1)
    if len(s) > 0:
        ans = 'No'
print(ans)