t = int(input())
s = input()
r = s[::-1]
s *= 2
i = 0

while i < len(s):
    t = 0
    while i < len(s) and s[i] == r[t]:
        t += 1
        i += 1
        if t == len(r):
            print(i - len(r))
            exit()
    i += 1

print(-1)