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: """
  // Author : snape_here - Susanta Mukherjee
       
   """
  
  from __future__ import division, print_function
   
  import os,sys
  from io import BytesIO, IOBase
   
  if sys.version_info[0] < 3:
      from __builtin__ import xrange as range
      from future_builtins import ascii, filter, hex, map, oct, zip
   
  def ii(): return int(input())
  def fi(): return float(input())
  def si(): return input()
  def msi(): return map(str,input().split())
  def mi(): return map(int,input().split())
  def li(): return list(mi())
  def lsi(): return list(msi())
   
  def read():
      sys.stdin = open('input.txt', 'r')  
      sys.stdout = open('output.txt', 'w') 
   
  def gcd(x, y):
      while y:
          x, y = y, x % y
      return x
  
  def lcm(x, y):
      return (x*y)//(gcd(x,y))
  
  mod=1000000007
  
  def modInverse(b,m): 
      g = gcd(b, m)  
      if (g != 1):         
          return -1
      else:          
          return pow(b, m - 2, m) 
  
  def ceil2(x,y):
      if x%y==0:
          return x//y
      else:
          return x//y+1
  
  def modu(a,b,m): 
  
      a = a % m 
      inv = modInverse(b,m) 
      if(inv == -1): 
          return -999999999
      else: 
          return (inv*a)%m
  
  from math import log,factorial,cos,tan,sin,radians,floor,sqrt,ceil
  
  import bisect
  import random
  import string 
  
  from decimal import *
  
  getcontext().prec = 50
  
  abc="abcdefghijklmnopqrstuvwxyz"
  
  pi=3.141592653589793238
  
  def gcd1(a):
      if len(a) == 1:
          return a[0]
      ans = a[0]
      for i in range(1,len(a)):
          ans = gcd(ans,a[i])
      return ans
  
  def mykey(x):
      return len(x)
  
  def main():
  
      for _ in range(ii()):
          n=ii()
          s=list(si())
          s1 = sorted(s)
          c = 0 
          for i in range(n):
              if s[i]!=s1[i]:
                  c += 1
          print(c)
  
  
  # region fastio
   
  BUFSIZE = 8192
   
  
  class FastIO(IOBase):
      newlines = 0
   
      def __init__(self, file):
          self._fd = file.fileno()
          self.buffer = BytesIO()
          self.writable = "x" in file.mode or "r" not in file.mode
          self.write = self.buffer.write if self.writable else None
   
      def read(self):
          while True:
              b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
              if not b:
                  break
              ptr = self.buffer.tell()
              self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
          self.newlines = 0
          return self.buffer.read()
   
      def readline(self):
          while self.newlines == 0:
              b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
              self.newlines = b.count(b"\n") + (not b)
              ptr = self.buffer.tell()
              self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
          self.newlines -= 1
          return self.buffer.readline()
   
      def flush(self):
          if self.writable:
              os.write(self._fd, self.buffer.getvalue())
              self.buffer.truncate(0), self.buffer.seek(0)
   
   
  class IOWrapper(IOBase):
      def __init__(self, file):
          self.buffer = FastIO(file)
          self.flush = self.buffer.flush
          self.writable = self.buffer.writable
          self.write = lambda s: self.buffer.write(s.encode("ascii"))
          self.read = lambda: self.buffer.read().decode("ascii")
          self.readline = lambda: self.buffer.readline().decode("ascii")
   
   
  def print(*args, **kwargs):
      """Prints the values to a stream, or to sys.stdout by default."""
      sep, file = kwargs.pop("sep", " "), kwargs.pop("file", sys.stdout)
      at_start = True
      for x in args:
          if not at_start:
              file.write(sep)
          file.write(str(x))
          at_start = False
      file.write(kwargs.pop("end", "\n"))
      if kwargs.pop("flush", False):
          file.flush()
   
   
  if sys.version_info[0] < 3:
      sys.stdin, sys.stdout = FastIO(sys.stdin), FastIO(sys.stdout)
  else:
      sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)
   
  input = lambda: sys.stdin.readline().rstrip("\r\n")
   
  # endregion
  
   
  if __name__ == "__main__":
      #read()
      main()