#! ruby
# Try yukicoder
# author: Leornadone @ NEETSDKASU

N = gets.to_i
S = gets.chomp

# S[i,j]
# S[j+1,j+(j+1-i)] = S[j+1,j+1+(j-i)]

m = Hash.new

N.pred.downto(0) do |i|
	N.pred.downto(i) do |j|
		k =S[i..j]
		if m.include? k
			b = (j + 1) + (j - i)
			if m[k].include? b
				puts "YES"
				exit
			end
			m[k][i] = true
		else
			m[k] = {i => true}
		end
	end
end

puts "NO"