N = int(input())
A = list(map(int, input().split()))
A = [10 ** 10] + A + [10 ** 10]
for i in range(1, N+1) :
	if A[i - 1] > A[i] and A[i] < A[i + 1] :
		if A[i] > 1 :
			print("No")
			exit()
print("Yes")