#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 f = 0;
  cerr << '{';
  for (auto &i : x) cerr << (f++ ? "," : ""), __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...);
}
const long long mod = 1e9 + 7;
const long long N = 4e5 + 5;
const long long INF = 1e18;
string s;
long long get(long long l, long long r, char c) {
  if (l == r) {
    return c != s[l];
  }
  long long m = (l + r) >> 1;
  long long ans1 = get(m + 1, r, c + 1);
  long long ans2 = get(l, m, c + 1);
  for (long long i = l; i <= m; i++) {
    ans1 += (c != s[i]);
  }
  for (long long i = m + 1; i <= r; i++) {
    ans2 += (c != s[i]);
  }
  return min(ans1, ans2);
}
void solve() {
  long long n;
  cin >> n;
  cin >> s;
  char c = 'a';
  cout << get(0, n - 1, c) << "\n";
}
int32_t main() {
  long long t;
  cin >> t;
  while (t--) {
    solve();
  }
}
