from __future__ import division, print_function
_interactive = False

def main():
    from itertools import product

    for _ in range(int(input())):
        n = int(input())
        ar = [ord(c)-ord('a') for c in input()]

        if n == 1:
            print(int(ar != [0]))
            continue

        h = n.bit_length()
        prefsums = []
        for i in range(h):
            ps = [0]
            for x in ar:
                ps.append(ps[-1] + (x==i))
            prefsums.append(ps)

        ans = INF
        for p in product((0, 1), repeat=h-1):
            p = list(p) + [1-p[-1]]
            l = n
            st = 0
            cur = 0
            for i in range(h):
                ps = prefsums[i]
                if i != h-1:
                    l //= 2
                if p[i] == 0:
                    cur += l - (ps[st+l]-ps[st])
                    if i != h-2:
                        st += l
                elif p[i] == 1:
                    cur += l - (ps[st+l+l]-ps[st+l])
            ans = min(ans, cur)

        print(ans)


# Constants
INF = float('inf')
MOD = 10**9+7
alphabets = 'abcdefghijklmnopqrstuvwxyz'

# Python3 equivalent names
import os, sys, itertools
if sys.version_info[0] < 3:
    input = raw_input
    range = xrange

    filter = itertools.ifilter
    map = itertools.imap
    zip = itertools.izip

# print-flush in interactive problems
if _interactive:
    flush = sys.stdout.flush
    def printf(*args, **kwargs):
        print(*args, **kwargs)
        flush()

# Debug print, only works on local machine
LOCAL = "LOCAL_" in os.environ
debug_print = (print) if LOCAL else (lambda *x, **y: None)

# Fast IO
if (not LOCAL) and (not _interactive):
    from io import BytesIO
    from atexit import register
    sys.stdin = BytesIO(os.read(0, os.fstat(0).st_size))
    sys.stdout = BytesIO()
    register(lambda: os.write(1, sys.stdout.getvalue()))
    input = lambda: sys.stdin.readline().rstrip('\r\n')

# Some utility functions(Input, N-dimensional lists, ...)
def input_as_list():
    return [int(x) for x in input().split()]

def input_with_offset(o):
    return [int(x)+o for x in input().split()]

def input_as_matrix(n, m):
    return [input_as_list() for _ in range(n)]

def array_of(f, *dim):
    return [array_of(f, *dim[1:]) for _ in range(dim[0])] if dim else f()

main()
