length = int(input())
string = input()
rev = "".join([ string[i] for i in range(len(string)-1, -1, -1)])
k=0
for i in range(0,len(string)):
    if string[i]==rev[0]:
        if rev==(string[i:]+string[:i]):
            print(i)
            break
else:
    print(-1)