n = int(input())
a = list(map(int, input().split()))

if not a:
    print("Yes")
    exit()

max_a = max(a)
if max_a == 0:
    print("Yes")
    exit()

max_m = 0
for i in range(n):
    left = a[i-1] if i > 0 else 0
    right = a[i+1] if i < n-1 else 0
    max_neighbor = max(left, right)
    if max_neighbor < a[i]:
        current_m = min(a[i], max_neighbor + 1)
        if current_m > max_m:
            max_m = current_m

print("Yes" if max_m >= max_a else "No")