
#------------------------------warmup----------------------------
# *******************************
# * AUTHOR:  RAJDEEP GHOSH      *   
# * NICK  :  Rajdeep2k          *  
# * INSTITUTION: IIEST, SHIBPUR *
# *******************************
import os
import sys
from io import BytesIO, IOBase
import math 
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")
 
#-------------------game starts now---------------------------------------------------
def chk(s,n,ch):
    if n==0:
        return 0
    elif n==1:
        if s[0]!=ch:
            return 1
        else:
            return 0
    a=0
    a1=0
    for i in range(0,n//2):
        if s[i]!=ch:
            a+=1
    a+= chk(s[n//2:],n//2,chr(ord(ch)+1))
    for i in range(n//2,n):
        if s[i]!=ch:
            a1+=1
    a1+=chk(s[0:n//2],n//2,chr(ord(ch)+1))
    ssss=set()

    return min(a,a1)
for _ in range((int)(input())):
    n=(int)(input())
    s=input()
    l=list()
    l.append(4)
    ss=set(l)
    res=1
    mm=1
    if n==1:
        if s=='a':
            print(0)
        else:
            print(1)
        continue
    ans=chk(s,n,'a')
    print(ans+res-mm)    