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: # import math
  # import collections
  # from itertools import permutations
  # from itertools import combinations
  # import sys
  # sys.stdin = open('input.txt', 'r')
  # sys.stdout = open('output.txt', 'w')
  
  
  '''def is_prime(n):
      j=2
      while j*j<=n:
          if n%j==0:
              return 0
          j+=1
      return 1'''
  
  
  '''def gcd(x, y):
      while(y):
          x, y = y, x % y
      return x'''
  
  '''fact=[]
  def factors(n) :
       
      i = 1
      while i <= math.sqrt(n):
           
          if (n % i == 0) :
               
              if (n / i == i) :
                  fact.append(i)
  
              else :
      
                  fact.append(i)
                  fact.append(n//i)
          i = i + 1'''
  
  
  
  def prob():
      
      n=int(input())
      s = input()
      ans=0
      prev=sorted(s)
  
      for i in range(n):
  
          if (prev[i]!=s[i]):
              ans+=1
  
      print(ans)
  
  
      
  
  t=1
  t=int(input())
  
  for _ in range(0,t):
      prob()