N = int(raw_input())
S = raw_input()
flag = False
if len(S) <= 3:
	for i in range(N):
		for j in range(i,N):
			if j + (j + 1 - i) > N:break
			if S[i - 1:j] == S[j:j + (j + 1 - i)]:
				flag = True
				break
		if flag:break
else:
	flag = True
print ('YES' if flag else 'NO')