from collections import Counter
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 dice():
    dp = [0] * (n + 1)
    dp[1] = 1
    dp[0] = 1
    for i in range(2, n + 1):
        for j in range(1, 7):
            if i - j >= 0:
                dp[i] = (dp[i] + dp[i - j]) % (10 ** 9 + 7)
            else:
                break


#for _ in range(int(input())):
#import math
import sys
# from collections import deque
#from collections import Counter
# ls=list(map(int,input().split()))
# for i in range(m):
# for i in range(int(input())):
#n,k= map(int, input().split())
#arr=list(map(int,input().split()))
#n=sys.stdin.readline()
#n=int(n)
# n = int(inaput())

def cost(l,r,ch,s):
    var=0
    for i in range(l,r):
        if s[i]!=ch:
            var+=1
    return var
def fxn(l,r,ch,s):
    if l+1==r:
        if s[l]==ch:
            return 0
        return 1
    mid=(l+r)//2
    return min(cost(l,mid,ch,s)+fxn(mid,r,chr(ord(ch)+1),s),fxn(l,mid,chr(ord(ch)+1),s)+cost(mid,r,ch,s))
for _ in range(int(input())):
    n = int(input())
    s=input()
    s=s
    ans=fxn(0,n,"a",s)
    print(ans)