#include <bits/stdc++.h>
using namespace std;
bool isPrime(long long n) {
  if (n <= 1) return false;
  if (n <= 3) return true;
  if (n % 2 == 0 || n % 3 == 0) return false;
  for (long long i = 5; i * i <= n; i = i + 6) {
    if (n % i == 0 || n % (i + 2) == 0) return false;
  }
  return true;
}
long long gcd(long long a, long long b) {
  if (a == 0) return b;
  if (a < b) swap(a, b);
  return gcd(a % b, b);
}
long long gcdextended(long long a, long long b, long long &x, long long &y) {
  if (a == 0) {
    x = 0;
    y = 1;
    return b;
  }
  long long x1, y1, gcd = gcdextended(b % a, a, x1, y1);
  x = y1 - (b / a) * x1;
  y = x1;
  return gcd;
}
long long modinverse(long long a, long long m) {
  long long x, y;
  long long g = gcdextended(a, m, x, y);
  if (g != 1) {
    cout << "NOT POSSIBLE";
    return -1;
  } else
    return (x % m + m) % m;
}
long long po(long long x, long long y) {
  long long res = 1;
  while (y > 0) {
    if (y & 1) res = (res * x);
    y = y >> 1;
    x = (x * x);
  }
  return res;
}
long long pom(long long x, long long y, long long p) {
  long long res = 1;
  x = x % p;
  while (y > 0) {
    if (y & 1) res = (res * x) % p;
    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}
long long root(long long x, vector<long long> &id) {
  while (id[x] != x) {
    id[x] = id[id[x]];
    x = id[x];
  }
  return x;
}
void uni(long long x, long long y, vector<long long> &id) {
  long long a = root(x, id);
  long long b = root(y, id);
  id[a] = id[b];
}
bool sec(const pair<long long, long long> &a,
         const pair<long long, long long> &b) {
  if (a.second == b.second) return a.first < b.first;
  return a.second < b.second;
}
long long ff(long long l, long long r, string &second, char c) {
  if (l == r) {
    if (second[l] == c) return 0;
    return 1;
  }
  long long i, m = (l + r) / 2, a = 0, b = 0;
  for (i = l; i < m + 1; i++)
    if (second[i] != c) a++;
  for (i = m + 1; i < r + 1; i++)
    if (second[i] != c) b++;
  return min(a + ff(m + 1, r, second, c + 1), b + ff(l, m, second, c + 1));
}
void solve() {
  long long n, i, j;
  cin >> n;
  string second;
  cin >> second;
  cout << ff(0, n - 1, second, 'a') << endl;
}
int main() {
  ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
  srand(time(NULL));
  ;
  long long t;
  t = 1;
  cin >> t;
  while (t--) {
    solve();
  }
}
