#include <bits/stdc++.h>
using namespace std;
double EPS = 1e-9;
long long INF = 1000000000000000005ll;
double PI = acos(-1);
inline string IntToString(long long int a) {
  char x[100];
  sprintf(x, "%lld", a);
  string s = x;
  return s;
}
inline long long int StringToInt(string a) {
  char x[100];
  long long int res;
  strcpy(x, a.c_str());
  sscanf(x, "%lld", &res);
  return res;
}
inline string uppercase(string s) {
  int n = (int)s.size();
  for (long long int i = 0; i < n; i++)
    if (s[i] >= 'a' && s[i] <= 'z') s[i] = s[i] - 'a' + 'A';
  return s;
}
inline string lowercase(string s) {
  int n = (int)s.size();
  for (long long int i = 0; i < n; i++)
    if (s[i] >= 'A' && s[i] <= 'Z') s[i] = s[i] - 'A' + 'a';
  return s;
}
bool cmp(pair<long long int, long long int> a,
         pair<long long int, long long int> b) {
  long long int as = a.first - a.second;
  long long int binary_search = b.first - b.second;
  return (as > binary_search);
}
long long int dx[] = {-1, 0, 1, 0};
long long int dy[] = {0, 1, 0, -1};
vector<long long int> v[100005];
vector<bool> vis(100005);
long long int ans = 0;
long long int solve(long long int l, long long int r, string s, char c) {
  long long int f = 0;
  long long int k = 0;
  long long int sum = 0;
  if (l + 1 == r) {
    if (s[l] == c) {
      return 0;
    } else {
      return ++sum;
    }
  }
  long long int mid = (r + l) / 2;
  string ls, rs;
  for (long long int i = l; i < mid; i++) {
    if (s[i] != c) f++;
    ls.push_back(s[i]);
  }
  for (long long int i = mid; i < r; i++) {
    if (s[i] != c) k++;
    rs.push_back(s[i]);
  }
  return min(f + solve(0, r / 2, rs, c + 1), k + solve(0, r / 2, ls, c + 1));
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int t;
  cin >> t;
  while (t--) {
    long long int n;
    cin >> n;
    string s;
    cin >> s;
    ans = 0;
    cout << solve(0, n, s, 'a') << '\n';
  }
  return 0;
}
