N = int(input())
A = list(map(int,input().split()))
B = [i+1 for i in range(N)]
C = [N-i for i in range(N)]

if(all(A[i] <= B[i] for i in range(N)) or all(A[i] <= C[i] for i in range(N))):
    print("Yes")
else:
    print("No")