N = int(input())
S = '%' + input().rstrip()
if N >= 4:
    print("YES")
    exit()
for i in range(1, N):
    j = i
    while j + (j + 1 - i) <= N:
        if S[i:j + 1] == S[j + 1:j + (j + 1 - i) + 1]:
            print("YES")
            exit()
        j += 1
print("NO")