N = int(input())
A = list(map(int, input().split()))
B = []
T = []
res = 'YES'
n = 1
while len(A) != 0:
    if n in A:
        k = A.index(n)
        B.append(n)
        N = A[:k + 1]
        for i in N:
            A.remove(i)
        T += N[:k]
        n += 1
    else:
        k = T.index(n)
        if k != len(T) - 1:
            res = 'NO'
            break
        else:
            T.pop()
            B.append(n)
            n += 1
if res == 'YES' and T == sorted(T, reverse = True):
    res = 'YES'
else:
    res = 'NO'
print(res)