#!/usr/bin/env python
from __future__ import division, print_function

import os
import 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

############################################################################
#                           ACTUAL CODE
############################################################################
def solve(t):
	from functools import lru_cache

	n = int(input())
	data = input()

	@lru_cache(None)
	def dp(i, j, a):
		if i == j:
			return 0 if data[i] == a else 1
		m = (j+i) // 2

		# checking the first-half
		f_nums = 0
		for k in range(i,m+1): 
			if data[k] != a: f_nums += 1

		l_nums = 0
		for k in range(m+1,j+1): 
			if data[k] != a: l_nums += 1

		return min(
			f_nums + dp(m+1, j, chr(ord(a)+1)),
			l_nums + dp(i, m, chr(ord(a)+1))
		)

	return dp(0, n-1, 'a')




def main():
	tc = int(input().strip())

	for t in range(1, tc+1):
		print(solve(t))

	# print(solve(1))

############################################################################
#                         FAST-IO
#                         PyRIVAL
############################################################################

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()

# ''' uncomment if fastIO not required
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")
# '''

############################################################################



## DRIVER # PROGRAM ########################################################

if __name__ == "__main__":
	main()
