# ------------------- fast io --------------------
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")
 
# ------------------- fast io --------------------

import math

testcases=int(input())

for j in range(testcases):
    n=int(input())
    vals=[ord(k)-97 for k in input()]
    #count the letters in the first half
    com1=0
    dict1=0
    cv1=n//2
    cv2=0
    checker=1
    left=0
    right=n-1
    while cv1>0:
        if checker==0:
            if vals[right-cv2]==com1:
                dict1+=1
        else:
            if vals[left+cv2]==com1:
                dict1+=1
        cv2+=1
        if cv2==cv1:
            if checker==0:
                right-=cv2
            else:
                left+=cv2
            com1+=1
            cv2=0
            cv1=cv1//2
            checker+=1
            checker=checker%2
    if vals[right]==com1:
        dict1+=1
    #######################
    dict2=0
    com1=0
    #
    cv1=n//2
    cv2=0
    checker=0
    left=0
    right=n-1
    while cv1>0:
        if checker==0:
            if vals[right-cv2]==com1:
                dict2+=1
        else:
            if vals[left+cv2]==com1:
                dict2+=1
        cv2+=1
        if cv2==cv1:
            if checker==0:
                right-=cv2
            else:
                left+=cv2
            com1+=1
            cv2=0
            cv1=cv1//2                  
            checker+=1
            checker=checker%2
    if vals[left]==com1:
        dict2+=1
    #find the most common letter in both halves and use the min?
    com1=0
    if dict1==n or dict2==n:
        print(0)
    else:
        moves=0
        if dict2<=dict1:
            com1=0
            cv1=n//2
            cv2=0
            checker=1
            left=0
            right=n-1
            while cv1>0:
                if checker==0:
                    if vals[right-cv2]!=com1:
                        moves+=1
                else:
                    if vals[left+cv2]!=com1:
                        moves+=1
                cv2+=1
                if cv2==cv1:
                    if checker==0:
                        right-=cv2
                    else:
                        left+=cv2
                    com1+=1
                    cv2=0
                    cv1=cv1//2
                    checker+=1
                    checker=checker%2
            if vals[right]!=com1:
                moves+=1
            print(moves)
        else:
            com1=0
            cv1=n//2
            cv2=0
            checker=0
            left=0
            right=n-1
            while cv1>0:
                if checker==0:
                    if vals[right-cv2]!=com1:
                        moves+=1
                else:
                    if vals[left+cv2]!=com1:
                        moves+=1
                cv2+=1
                if cv2==cv1:
                    if checker==0:
                        right-=cv2
                    else:
                        left+=cv2
                    com1+=1
                    cv2=0
                    cv1=cv1//2                  
                    checker+=1
                    checker=checker%2
            if vals[left]!=com1:
                moves+=1
            print(moves)