N = int(input())
S = input()
flag = 0
if len(S) > 3:
    print("YES")
    flag = 1
else:    
    for i in range(len(S)):
        for j in range(i, len(S)):
            try:
                a = S[i - 1:j]
                b = S[j : j + (j + 1 - i)]
            except:
                break
            if a == b:
                print("YES")
                flag = 1
                break
        if flag == 1:
            break

if flag != 1:
    print("NO")