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 j + 1 >= N:break
		if S[i:j] == S[j + 1:j + (j + 1- i)]:
			flag = True
	if flag:
		break
print ('YES' if flag else 'NO')