N = input()
S = list(raw_input())

if N <= 1:
    print "NO"

else:
    for i in xrange(N-1):
        for w in xrange(1, N-i):
            if S[i:i+w+1] == S[i+w+1:i+2*w+1+1]:
                print "YES"
                exit()
    
    else: print "NO"