#include <bits/stdc++.h>
using namespace std;
const long long N = pow(2, 32);
map<long long, long long> factorize(long long n) {
  map<long long, long long> ans;
  for (long long i = 2; i * i <= n; i++) {
    while (n % i == 0) {
      ans[i]++;
      n /= i;
    }
  }
  if (n > 1) {
    ans[n]++;
    n = 1;
  }
  return ans;
}
void FAST() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
}
long long binpow(long long a, long long b, long long m) {
  a %= m;
  long long res = 1;
  while (b > 0) {
    if (b & 1) res = res * a % m;
    a = a * a % m;
    b >>= 1;
  }
  return res;
}
string s;
long long answer(char c, long long l, long long r) {
  if (l == r) {
    if (s[l] != c)
      return 1;
    else
      return 0;
  }
  long long ans1 = 0, ans2 = 0;
  long long mid = (l + r) / 2;
  ans1 += mid - l - count(s.begin() + l, s.begin() + mid + 1, c) + 1;
  ans2 += r - mid - count(s.begin() + mid + 1, s.begin() + r + 1, c);
  ans1 += answer(c + 1, mid + 1, r);
  ans2 += answer(c + 1, l, mid);
  return min(ans1, ans2);
}
int main() {
  FAST();
  long long t;
  cin >> t;
  while (t--) {
    long long n;
    cin >> n;
    cin >> s;
    cout << answer('a', 0, n - 1) << "\n";
  }
}
