description: A string s of length n, consisting of lowercase letters of the English alphabet, is given.
  
  You must choose some number k between 0 and n. Then, you select k characters of s and permute them however you want. In this process, the positions of the other n-k characters remain unchanged. You have to perform this operation exactly once.
  
  For example, if s="andrea", you can choose the k=4 characters "a_d_ea" and permute them into "d_e_aa" so that after the operation the string becomes "dneraa".
  
  Determine the minimum k so that it is possible to sort s alphabetically (that is, after the operation its characters appear in alphabetical order).
  
  Input
  
  The first line contains a single integer t (1 ≤ t ≤ 1000) — the number of test cases. Then t test cases follow.
  
  The first line of each test case contains one integer n (1 ≤ n ≤ 40) — the length of the string.
  
  The second line of each test case contains the string s. It is guaranteed that s contains only lowercase letters of the English alphabet.
  
  Output
  
  For each test case, output the minimum k that allows you to obtain a string sorted alphabetically, through the operation described above.
  
  Example
  
  Input
  
  
  4
  3
  lol
  10
  codeforces
  5
  aaaaa
  4
  dcba
  
  
  Output
  
  
  2
  6
  0
  4
  
  Note
  
  In the first test case, we can choose the k=2 characters "_ol" and rearrange them as "_lo" (so the resulting string is "llo"). It is not possible to sort the string choosing strictly less than 2 characters.
  
  In the second test case, one possible way to sort s is to consider the k=6 characters "_o__force_" and rearrange them as "_c__efoor_" (so the resulting string is "ccdeefoors"). One can show that it is not possible to sort the string choosing strictly less than 6 characters.
  
  In the third test case, string s is already sorted (so we can choose k=0 characters).
  
  In the fourth test case, we can choose all k=4 characters "dcba" and reverse the whole string (so the resulting string is "abcd").
solution: from collections import Counter, defaultdict, deque
  import bisect
  from sys import stdin, stdout
  from itertools import repeat, permutations
  import math
  import heapq
  
  MOD = 998244353
  def inp(force_list=False):
      re = map(int, raw_input().split())
      if len(re) == 1 and not force_list:
          return re[0]
      return re
  
  def inpf(force_list=False):
      re = map(float, raw_input().split())
      if len(re) == 1 and not force_list:
          return re[0]
      return re
  
  def inst():
      return raw_input().strip()
  
  def gcd(x, y):
     while(y):
         x, y = y, x % y
     return x
  
  def lcm(x, y):
      return x*y/(gcd(x,y))
  
  def qmod(a, b, mod=MOD):
      res = 1
      while b:
          if b&1:
              res = (res*a)%mod
          b >>= 1
          a = (a*a)%mod
          # print b
      return res
  
  def inv(a):
      return qmod(a, MOD-2)
  
  def query(num):
      print num
      stdout.flush()
      return inp()
  
  def my_main():
      kase = inp()
      pans = []
      cur = []
      for _ in range(kase):
          n = inp()
          st = inst()
          sst = sorted(st)
          ans = 0
          for i in range(n):
              if st[i] != sst[i]:
                  ans += 1
          pans.append(str(ans))
      print '\n'.join(pans)
  
  
  my_main()
  