n=input()
s=raw_input()
def mys(s,a,b):
	return s[a-1:b]
if n>3:
	print"YES"
else:
	for i in range(1,n+1):
		for j in range(i,n+1):
			if j+j+1-i<=n and mys(s,i,j)==mys(s,j+1,j+j+1-i):
				print"YES"
				exit()
	print"NO"