#include <bits/stdc++.h>
using namespace std;
template <class T>
ostream &operator<<(ostream &os, const vector<T> &V) {
  os << "[ ";
  for (auto v : V) os << v << " ";
  return os << "]";
}
template <class T>
ostream &operator<<(ostream &os, const set<T> &S) {
  os << "{ ";
  for (auto s : S) os << s << " ";
  return os << "}";
}
template <class T>
ostream &operator<<(ostream &os, const unordered_set<T> &S) {
  os << "{ ";
  for (auto s : S) os << s << " ";
  return os << "}";
}
template <class L, class R>
ostream &operator<<(ostream &os, const pair<L, R> &P) {
  return os << "(" << P.first << "," << P.second << ") ";
}
template <class L, class R>
ostream &operator<<(ostream &os, const map<L, R> &M) {
  os << "{ ";
  for (auto m : M) os << "(" << m.first << ":" << m.second << ") ";
  return os << "}";
}
template <class T>
ostream &operator<<(ostream &os, stack<T> S) {
  while (!S.empty()) {
    os << S.top() << "\n";
    S.pop();
  }
  return os;
}
template <class T>
ostream &operator<<(ostream &os, queue<T> Q) {
  while (!Q.empty()) {
    os << Q.front() << " ";
    Q.pop();
  }
  return os;
}
const long long mod = 1e9 + 7;
inline long long modulo(long long x) {
  x = x % mod;
  return x < 0 ? x + mod : x;
}
inline long long add(long long x, long long y) {
  x += y;
  return modulo(x);
}
inline long long sub(long long x, long long y) {
  x -= y;
  return modulo(x);
}
inline long long mul(long long x, long long y) {
  x = x * 1ll * y;
  return modulo(x);
}
inline long long power(long long x, long long y) {
  if (y == 0) return 1;
  long long temp = power(x, y / 2);
  temp = mul(temp, temp);
  if (y % 2 == 1) temp = mul(temp, x);
  return temp;
}
inline long long inv(long long x) { return power(x, mod - 2); }
long long makeall(vector<char> &cache, long long i, long long j, char c) {
  long long count = 0;
  for (long long k = i; i < j ? k <= j : k >= j; i < j ? k++ : k--) {
    if (cache[k] != c) count++;
  }
  return count;
}
long long make(vector<char> &cache, long long i, long long j, char c) {
  if (i == j) return (cache[i] == c) ? 0 : 1;
  long long mid = (i + j) / 2;
  long long temp = makeall(cache, i, mid, c) + make(cache, mid + 1, j, c + 1);
  temp = min(makeall(cache, mid + 1, j, c) + make(cache, i, mid, c + 1), temp);
  return temp;
}
void solve() {
  long long n;
  cin >> n;
  vector<char> cache(n, '@');
  for (long long i = 0; i < n; i++) cin >> cache[i];
  cout << make(cache, 0, n - 1, 'a') << endl;
}
int32_t main() {
  long long T = 1;
  cin >> T;
  for (long long t = 1; 1 < T ? t <= T : t >= T; 1 < T ? t++ : t--) {
    solve();
  }
  return 0;
}
