#include <iostream>

#include <vector>

#include <string>

using namespace std;

int T, N;

string str;

vector<int> ans;

int min(int a, int b) {

if (a <= b) return a;

return b;

}

int solve(int left, int right, char currentChar, int currentNum) {

if (currentNum == 1) {

if (str[left] == currentChar) return 0;

else return 1;

}

int middle = (left + right) / 2;

int count1 = 0, count2 = 0;

int i;

for (i = left; i <= middle; i++) {

if (currentChar != str[i]) count1++;

}

for (i = middle + 1; i <= right; i++) {

if (currentChar != str[i])count2++;

}

int res1 = solve(middle + 1, right, currentChar + 1, currentNum / 2) + count1;

int res2 = solve(left, middle, currentChar + 1, currentNum / 2) + count2;

return min(res1, res2);

}

int main()

{

cin >> T;

int i;

for (i = 0; i < T; i++) {

cin >> N >> str;

int a = solve(0, N - 1, 'a', N);

ans.push_back(a);

}

for (i = 0; i < T; i++) {

cout << ans[i] << "\n";

}

return 0;

}