#! 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)]

N.times do |i|
	N.times do |j|
		b2 = j + (j + 1 - i)
		break if b2 > N
		if S[i..j] == S[j+1,b2]
			puts "YES"
			exit
		end
	end
end

puts "NO"