N = int(input())
A = list(map(int, input().split()))
B = [10 ** 10, A[0]]
for i in range(1, N) :
	if A[i] != A[i - 1] :
		B.append(A[i])
B.append(10 ** 10)
for i in range(1, len(B) - 1) :
	if B[i - 1] > B[i] and B[i] < B[i + 1] :
		if B[i] > 1 :
			print("No")
			exit()
print("Yes")