import os
import sys
from io import BytesIO, IOBase

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

def list2d(a, b, c): return [[c] * b for i in range(a)]
def list3d(a, b, c, d): return [[[d] * c for j in range(b)] for i in range(a)]
def list4d(a, b, c, d, e): return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]
def ceil(x, y=1): return int(-(-x // y))
def Yes(): print('Yes')
def No(): print('No')
def YES(): print('YES')
def NO(): print('NO')
INF = 10 ** 18
MOD = 10**9+7

from math import log2

Ri = lambda : [int(x) for x in sys.stdin.readline().split()]
ri = lambda : sys.stdin.readline().strip()

for _ in range(int(ri())):
    n=  int(ri())
    s = ri()

    dp=  list2d(26, n+1, 0)
    for i in range(26):
        for j in range(n):
            if ord(s[j])-ord('a') == i:
                dp[i][j+1] = dp[i][j]+1
            else:
                 dp[ i][j+1] = dp[i][j]
    # print(dp[0][1], dp[0][0])
    # logg = ceil(log2(len(s)))
    # print(logg)
    # adp = list3d(len(s), logg+1, 26, -1)

    def solve(sta, lenn, ch):
        if lenn == 0:
            if ord(s[sta])-ord('a') == ch:
                return 0
            else:
                return 1

        end = sta+2**lenn-1
        mid= (sta+end)//2
        tlen = (mid - sta+1)
        # print(tlen-(dp[ch][mid+1] - dp[ch][sta]))
        ret =  min( solve(sta,lenn-1,ch+1) + tlen-(dp[ch][end+1] - dp[ch][mid+1]), 
                                 solve(sta+2**(lenn-1),lenn-1,ch+1) + tlen-(dp[ch][mid+1] - dp[ch][sta]) )
        # print(sta,end,ch, adp[sta][lenn][ch])
        return ret

    # print(adp)

    ret = solve(0, int(log2(n)), 0)
    print(ret)
    

