func main()
	var ans: bool :: false
	var t: []char :: cui@input()
	if(t =& null)
		do cui@print("NO\n")
		ret
	end if
	for i(0, ^t - 2)
		for j(i + 1, ^t - 1)
			var s: []char :: ##t
			for x(0, ^s - 1)
				if(x < i | j < x | (x - i) % 2 = 1)
					var c: char :: s[x]
					for y(0, ^s - 1)
						if(s[y] = c)
							do s[y] :: ' '
						end if
					end for
				end if
			end for
			var f: bool :: true
			for x(0, ^s - 1)
				if(i <= x & x <= j)
					if((x - i) % 2 = 0)
						if(s[x] = ' ')
							do f :: false
						end if
					else
						if(s[x] <> ' ')
							do f :: false
						end if
					end if
				end if
			end for
			if(f)
				do ans :: true
			end if
		end for
	end for
	do cui@print(ans ?("Yes\n", "NO\n"))
end func