#include <bits/stdc++.h>
using namespace std;
void fs(long long int &number) {
  bool negative = false;
  register long long int c;
  number = 0;
  c = getchar();
  if (c == '-') {
    negative = true;
    c = getchar();
  }
  for (; (c > 47 && c < 58); c = getchar()) number = number * 10 + c - 48;
  if (negative) number *= -1;
}
long long int power(long long int a, long long int x) {
  a %= 1000000007;
  long long int res = 1;
  while (x > 0) {
    if (x & 1) res = res * a % 1000000007;
    a = a * a % 1000000007;
    x >>= 1;
  }
  return res;
}
long long int max(long long int a, long long int b) {
  if (a > b) {
    return a;
  } else {
    return b;
  }
}
long long int min(long long int a, long long int b) {
  if (a > b) {
    return b;
  } else {
    return a;
  }
}
long long int madd(long long int a, long long int b) {
  long long int ans;
  ans = (a % 1000000007 + b % 1000000007) % 1000000007;
  return ans;
}
long long int mmult(long long int a, long long int b) {
  long long int ans = 1;
  ans = ((a % 1000000007) * (b % 1000000007)) % 1000000007;
  return ans % 1000000007;
}
long long int modinverse(long long int a) { return power(a, 1000000007 - 2); }
string sub(string s, long long int start, long long int end) {
  string temp = "";
  for (long long int i = start; i <= end; i++) {
    temp += s[i];
  }
  return temp;
}
long long int recurse(string s, long long int id) {
  if (s.length() == 1) {
    if (s[0] == 'a' + id) {
      return 0;
    } else {
      return 1;
    }
  }
  long long int x = s.length() / 2;
  string s1 = sub(s, 0, x - 1);
  string s2 = sub(s, x, s.length() - 1);
  char rc = 'a' + id;
  long long int c1 = 0, c2 = 0;
  for (long long int i = 0; i < s1.length(); i++) {
    if (s1[i] != rc) {
      c1++;
    }
  }
  for (long long int i = 0; i < s2.length(); i++) {
    if (s2[i] != rc) {
      c2++;
    }
  }
  return min(c1 + recurse(s2, id + 1), c2 + recurse(s1, id + 1));
}
void its_her_choice() {
  long long int n;
  cin >> n;
  string s;
  cin >> s;
  cout << recurse(s, 0);
  cout << "\n";
}
signed main() {
  long long int t;
  cin >> t;
  for (long long int casn = 1; casn < t + 1; casn++) {
    its_her_choice();
  }
}
