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

if N <= 1:
    print "NO"

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