N = int(input())
if N >= 4:
    print("YES")
elif N == 3:
    S = input()
    if S == "101" or S == "010":
        print("NO")
    else:
        print("YES")
elif N == 2:
    S = input()
    if S == "01" or S == "10":
        print("NO")
    else:
        print("YES")
else:
    print("NO")