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

import itertools, os, sys, threading
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


# 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)


from math import sqrt, floor, factorial, gcd, log, ceil
from collections import deque, Counter, defaultdict
from itertools import permutations, combinations
from math import gcd
from bisect import bisect
from random import randrange

# sys.setrecursionlimit(10**7)

input = lambda: sys.stdin.readline().rstrip("\r\n")
read = lambda: list(map(int, input().strip().split(" ")))

def main():
    for _ in range(int(input())):
        n = int(input()); s = input()
        def getCost(l, r, ch):
            return(r-l - s[l:r].count(chr(ch)))
        def getMin(l, r, ch):
            if l+1 == r:
                return([0, 1][chr(ch) != s[l]])
            return(min(getCost(l, (l+r)//2, ch)+getMin((l+r)//2, r, ch+1), getCost((l+r)//2, r, ch)+getMin(l, (l+r)//2, ch+1)))
        print(getMin(0, n, 97))













    








if __name__ == "__main__":
    main()
    # print(squer_gen(3))
    # sys.setrecursionlimit(100000000)
    # threading.stack_size(20480000)
    # thread = threading.Thread(target=main)
    # thread.start()



