# ------------------- 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 --------------------

testcases=int(input())

for j in range(testcases):
    n=int(input())
    vals=[ord(k)-97 for k in input()]
    cv=0
    moves=0
    allmoves=set([])
    def divncon(seg,cv,moves,allmoves):
        if len(seg)==1:
            if seg[0]!=cv:
                allmoves.add(moves+1)
                return allmoves
            else:
                allmoves.add(moves)
                return allmoves
        else:
            movel=moves
            mover=moves
            left=seg[:len(seg)//2]
            right=seg[len(seg)//2:len(seg)]
            for s in range(len(seg)//2):
                if left[s]!=cv:
                    movel+=1
                if right[s]!=cv:
                    mover+=1
            divncon(left,cv+1,mover,allmoves)
            divncon(right,cv+1,movel,allmoves)
            return allmoves
    moves=divncon(vals,0,moves,allmoves)
    print(min(moves))