## https://yukicoder.me/problems/no/2018


def main():
    N = int(input())
    S = input()
    T = input()

    if S[0] != T[0]:
        print(-1)
        return


    answer = 0
    for i in range(1, N - 1):
        if S[i] == T[i]:
            continue
        else:
            if T[i - 1] == S[i + 1]:
                answer += 1
            else:
                print(-1)
                return
    
    if S[-1] != T[-1]:
        print(-1)
        return
    print(answer)















if __name__ == "__main__":
    main()