func main()
	var n: int :: cui@inputInt()
	var s: []char :: cui@input()
	var ans: bool :: n > 3
	if(n = 1)
		do ans :: false
	elif(n = 2)
		do ans :: !(s = "01" | s = "10")
	elif(n = 3)
		do ans :: !(s = "010" | s = "101")
	end if
	do cui@print(ans ?("YES\n", "NO\n"))
end func