a=int(input())
slovo=input()
count=0
slovo2=slovo[::-1]
if slovo==slovo2:
    count=0
else:
    while slovo!=slovo2:
        if count>=a:
            count=-1
            break
        word=slovo[:1]
        slovo=slovo[1:]
        slovo+=word
        count+=1
print(count)