import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

N = int(readline())
S = read().rstrip().decode()

if N > 4:
    print('YES')
    exit()

answer = 'NO'
for i in range(N):
    for j in range(i,N):
        if S[i:j+1] == S[j+1:j-i+2]:
            answer = 'YES'

print(answer)