import time,math as mt,bisect,sys
sys.setrecursionlimit(132000*2)
from sys import stdin,stdout
from collections import deque
from fractions import Fraction
from collections import Counter
from collections import OrderedDict
pi=3.14159265358979323846264338327950
def II(): # to take integer input
    return int(stdin.readline())
def IO(): # to take string input
    return stdin.readline()
def IP(): # to take tuple as input
    return map(int,stdin.readline().split())
def L(): # to take list as input
    return list(map(int,stdin.readline().split()))
def P(x): # to print integer,list,string etc..
    return stdout.write(str(x)+"\n")
def PI(x,y): # to print tuple separatedly
    return stdout.write(str(x)+" "+str(y)+"\n")
def lcm(a,b): # to calculate lcm
    return (a*b)//gcd(a,b)
def gcd(a,b): # to calculate gcd
    if a==0:
        return b
    elif b==0:
        return a
    if a>b:
        return gcd(a%b,b)
    else:
        return gcd(a,b%a)
def readTree(): # to read tree
    v=int(input())
    adj=[set() for i in range(v+1)]
    for i in range(v-1):
        u1,u2=In()
        adj[u1].add(u2)
        adj[u2].add(u1)
    return adj,v
def bfs(adj,v): # a schema of bfs
    visited=[False]*(v+1)
    q=deque()
    while q:
        pass
def sieve():
    li=[True]*1000001
    li[0],li[1]=False,False
    for i in range(2,len(li),1):
        if li[i]==True:
            for j in range(i*i,len(li),i):
                li[j]=False
    prime=[]
    for i in range(1000001):
        if li[i]==True:
            prime.append(i)
    return prime
def setBit(n):
    count=0
    while n!=0:
        n=n&(n-1)
        count+=1
    return count
mx=10**7
spf=[mx]*(mx+1)
def SPF():
    spf[1]=1
    for i in range(2,mx+1):
        if spf[i]==mx:
            spf[i]=i
            for j in range(i*i,mx+1,i):
                if i<spf[j]:
                    spf[j]=i
    return
#####################################################################################
mod=10**9+7
def good(s,c,st,n):
    #print(s,c,st,n)
    if (n-st)==1:
        if s[st]!=c:
            return 1
        else:
            return 0
    else:
        mv1,mv2=0,0
        for i in range(st,st+(n-st)//2):
            if s[i]!=c:
                mv1+=1
        for i in range(st+(n-st)//2,n):
            if s[i]!=c:
                mv2+=1
        #print(mv1,mv2)
        c1=good(s,chr(ord(c)+1),st+(n-st)//2,n)
        c2=good(s,chr(ord(c)+1),st,st+(n-st)//2)
        if (mv1+c1)<(mv2+c2):
            return (mv1+c1)
        else:
            return (mv2+c2)

def solve():
    n=int(input())
    s=input()
    print(good(s,'a',0,n))

t=int(input())
for i in range(t):
    solve()

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