#include <bits/stdc++.h>
using namespace std;
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }
template <typename T, typename V>
void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T>
void __print(const T &x) {
  int first = 0;
  cerr << '{';
  for (auto &i : x) cerr << (first++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V>
void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v)) cerr << ", ";
  _print(v...);
}
int calc(const string &s, char c) {
  if (s.length() == 1) {
    return s[0] != c;
  }
  int mid = s.size() / 2;
  int cntl = calc(string(s.begin(), s.begin() + mid), c + 1);
  cntl += s.size() / 2 - count(s.begin() + mid, s.end(), c);
  int cntr = calc(string(s.begin() + mid, s.end()), c + 1);
  cntr += s.size() / 2 - count(s.begin(), s.begin() + mid, c);
  return min(cntl, cntr);
}
int main() {
  std::ios::sync_with_stdio(false);
  cin.tie(NULL);
  int T;
  cin >> T;
  while (T--) {
    int n;
    cin >> n;
    string s;
    cin >> s;
    int ans = calc(s, 'a');
    cout << ans << "\n";
  }
  return 0;
}
