#!/usr/bin/env python
import os
import operator
import sys
from io import BytesIO, IOBase

def inar():
    return [int(k) for k in input().split()]
def recur(prefix,suffix,char):
    ln=len(prefix)
    ans=0
    if ln==1:
        if ord(prefix[0])-97!=char:
            ans+=1
        if ord(suffix[0])-97!=char+1:
            ans+=1
        return ans
    for i in range(ln):
        if ord(prefix[i])-97!=char:
            ans+=1
    st1=suffix[0:ln//2]
    st2=suffix[ln//2:ln]
    res=min(recur(st1,st2,char+1),recur(st2,st1,char+1))
    return ans+res






def main():
    global st,prefix,suffix,ans,lev
    for _ in range(int(input())):
        n=int(input())
        # n,k=map(int,input().split())
        st=input()
        if n==1:
            if st[0]=="a":
                print(0)
            else:
                print(1)
            continue
        prefix=st[0:n//2]
        suffix=st[n//2:n]
        result=min(recur(prefix,suffix,0),recur(suffix,prefix,0))
        print(result)











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")

if __name__ == "__main__":
    main()