from math import inf, log2


class SegmentTree:
    def __init__(self, array, func=max):
        self.n = len(array)
        self.size = 2**(int(log2(self.n-1))+1) if self.n != 1 else 1
        self.func = func
        self.default = 0 if self.func != min else inf
        self.data = [self.default] * (2 * self.size)
        self.process(array)

    def process(self, array):
        self.data[self.size : self.size+self.n] = array
        for i in range(self.size-1, -1, -1):
            self.data[i] = self.func(self.data[2*i], self.data[2*i+1])

    def query(self, alpha, omega):
        """Returns the result of function over the range (inclusive)!"""
        if alpha == omega:
            return self.data[alpha + self.size]
        res = self.default
        alpha += self.size
        omega += self.size + 1
        while alpha < omega:
            if alpha & 1:
                res = self.func(res, self.data[alpha])
                alpha += 1
            if omega & 1:
                omega -= 1
                res = self.func(res, self.data[omega])
            alpha >>= 1
            omega >>= 1
        return res

    def update(self, index, value):
        """Updates the element at index to given value!"""
        index += self.size
        self.data[index] = value
        index >>= 1
        while index:
            self.data[index] = self.func(self.data[2*index], self.data[2*index+1])
            index >>= 1


# ------------------- 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 --------------------
from math import gcd, ceil

def pre(s):
    n = len(s)
    pi = [0] * n
    for i in range(1, n):
        j = pi[i - 1]
        while j and s[i] != s[j]:
            j = pi[j - 1]
        if s[i] == s[j]:
            j += 1
        pi[i] = j
    return pi


def prod(a):
    ans = 1
    for each in a:
        ans = (ans * each)
    return ans

def lcm(a, b): return a * b // gcd(a, b)


def binary(x, length=16):
    y = bin(x)[2:]
    return y if len(y) >= length else "0" * (length - len(y)) + y


for _ in range(int(input()) if True else 1):
    n = int(input())
    k = int(log2(n))
    #n, k = map(int, input().split())
    #a, b = map(int, input().split())
    #c, d = map(int, input().split())
    #a = list(map(int, input().split()))
    #b = list(map(int, input().split()))
    s = input()
    if n == 1:
        print(0 if s == "a" else 1)
        continue
    #print("YES" if s else "NO")
    fu = [[0]*n for ___ in range(26)]
    for i in range(n):
        fu[ord(s[i])-97][i] += 1
    for i in range(26):
        fu[i] = SegmentTree(fu[i], func=lambda a,b:a+b)
    ans = n
    for i in range(n):
        b = binary(i, k)
        p = 0
        q = n-1
        ind = -1
        anss = 0
        for c in b:
            ind += 1
            mid = (p+q) // 2
            #print(p, q)
            if c == '0':
                anss += (q-(mid+1)+1) - fu[ind].query(mid+1, q)
                q = mid
            else:
                anss += (mid-p+1) - fu[ind].query(p, mid)
                p = mid+1
        if fu[ind+1].query(q,q) != 1:anss+=1
        ans = min(ans, anss)
    print(ans)