human
A wise man told Kerem "Different is good" once, so Kerem wants all things in his life to be different. 

Kerem recently got a string s consisting of lowercase English letters. Since Kerem likes it when things are different, he wants all substrings of his string s to be distinct. Substring is a string formed by some number of consecutive characters of the string. For example, string "aba" has substrings "" (empty substring), "a", "b", "a", "ab", "ba", "aba".

If string s has at least two equal substrings then Kerem will change characters at some positions to some other lowercase English letters. Changing characters is a very tiring job, so Kerem want to perform as few changes as possible.

Your task is to find the minimum number of changes needed to make all the substrings of the given string distinct, or determine that it is impossible.

Input

The first line of the input contains an integer n (1 ≤ n ≤ 100 000) — the length of the string s.

The second line contains the string s of length n consisting of only lowercase English letters.

Output

If it's impossible to change the string s such that all its substring are distinct print -1. Otherwise print the minimum required number of changes.

Examples

Input

2
aa

Output

1

Input

4
koko

Output

2

Input

5
murat

Output

0

Note

In the first sample one of the possible solutions is to change the first character to 'b'.

In the second sample, one may change the first character to 'a' and second character to 'b', so the string becomes "abko".
The above is tricky. Write me a correct solution in PYTHON3.
 ... Give me a Python code to find the minimum number of changes needed to make all the substrings of the given string distinct, or determine that it is impossible?
assistant
#Here is Python code to find the minimum number of changes needed to make all the substrings of the given string distinct, or determine that it is impossible.

#!/usr/bin/env python3
# 672B_good.py - Codeforces.com/problemset/problem/672/B by Sergey 2016

import unittest
import sys

###############################################################################
# Good Class (Main Program)
###############################################################################

class Good:
    """ Good representation """

    def __init__(self, test_inputs=None):
        """ Default constructor """

        it = iter(test_inputs.split("\n")) if test_inputs else None

        def uinput():
            return next(it) if it else sys.stdin.readline().rstrip()

        # Reading single elements
        [self.n] = map(int, uinput().split())

        self.s = list(uinput())

    def calculate(self):
        """ Main calcualtion function of the class """

        freq = [0]*26
        for c in self.s:
            ltr = ord(c) - ord('a')
            freq[ltr] += 1

        zeroes = 0
        extra = 0
        for n in freq:
            if n == 0:
                zeroes += 1
            if n > 1:
                extra += n - 1

        result = -1 if extra > zeroes else extra

        return str(result)

###############################################################################
# Unit Tests
###############################################################################

class unitTests(unittest.TestCase):

    def test_single_test(self):
        """ Good class testing """

        # Constructor test
        test = "2\naa"
        d = Good(test)
        self.assertEqual(d.n, 2)
        self.assertEqual(d.s, ["a", "a"])

        # Sample test
        self.assertEqual(Good(test).calculate(), "1")

        # Sample test
        test = "4\nkoko"
        self.assertEqual(Good(test).calculate(), "2")

        # Sample test
        test = "5\nmurat"
        self.assertEqual(Good(test).calculate(), "0")

        # My tests
        test = "30\ndasdsadasdasdasdsadasdsadasdasda"
        self.assertEqual(Good(test).calculate(), "-1")

        # Time limit test
        #...