import os
import sys
from io import BytesIO, IOBase
import math
from decimal import Decimal
from decimal import *
from collections import defaultdict, deque
import heapq
getcontext().prec = 25
abcd='abcdefghijklmnopqrstuvwxyz'
MOD = pow(10, 9) + 7
BUFSIZE = 8192
from bisect import bisect_left, bisect_right
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")

sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)
input = lambda: sys.stdin.readline().rstrip("\r\n")

# n, k = map(int, input().split(" "))
# list(map(int, input().split(" ")))
# for _ in range(int(input())):


def solve(s, start, end, c, v, idd):
    mid = ((start+ end)//2)
    c = chr(ord(c)+ 1)
    if idd > n:
        x.append(v)
        return
    solve(s, start, mid, c, v + s[mid:end].count(c), idd*2)
    solve(s, mid, end, c, v + s[start:mid].count(c), idd*2)

for _ in range(int(input())):
    n = int(input())
    s = input()
    x = []
    v = 0
    idd = 1
    c = chr(96)
    solve(s,0,n,c,v, idd)
    print(n - max(x))

