N = int(raw_input())
S = raw_input()
flag = False
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
print ('YES' if flag else 'NO')