input()
s = input()
def www(qqq):
	b = {0:0}
	for i in range(1, len(qqq)):
		j = b[i - 1]
		while j > 0 and qqq[j] != qqq[i]:
			j = b[j - 1]
		if qqq[j] == qqq[i]:
			j = j+1
		b[i] = j
	return b

def sss(k, qqq):
	b = www(qqq)
	i = 0
	j = 0
	while i< len(k) and j < len(qqq):
		if qqq[j] == k[i]:
			i = i+1
			j = j+1
		elif j == 0:
			i = i+1
		else:
			j = b[j - 1]
	else:
		if j == len(qqq):
			return i - j
		return -1

print(sss(s*2, s[::-1]))