#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
#define ll long long
#define OO 2'000'000'000
#define ull unsigned long long
#define nl '\n'
#define sz(x) (ll)(x.size())
#define all(x) x.begin(),x.end()
#define rall(s)  s.rbegin(), s.rend()
#define getline(s) getline(cin>>ws,s)
#define ceill(n, m) (((n) / (m)) + ((n) % (m) ? 1 : 0))
#define pi  3.141592653589793
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
#define multi_ordered_set tree<int, null_type,less_equal<int>, rb_tree_tag,tree_order_statistics_node_update>


void Fast_IO(){
ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
// freopen("filename.in", "r", stdin);
// freopen("filename.txt", "w", stdout);
#ifndef ONLINE_JUDGE
freopen("input.txt", "r", stdin), freopen("output.txt", "w", stdout);
#endif
}




int dx[] = { 2, 1, -1, -2, -2, -1, 1, 2 };
int dy[] = { 1, 2, 2, 1, -1, -2, -2, -1 };

ll calc(string s, char c){
    ll ans = 0;
    for(auto&i:s) ans+=(i!=c);
    return ans;
}

ll min_operation(string s,char c){
    if(sz(s)==1) return s[0] == c ?0:1;
    ll mid = sz(s)/2;
    ll op1 = calc(s.substr(0,mid),c) + min_operation(s.substr(mid),c+1);
    ll op2 = calc(s.substr(mid),c) + min_operation(s.substr(0,mid),c+1);
    return min(op1,op2);
}




void solve(){
ll n; cin>>n;
string s; cin>>s;
cout<<min_operation(s,'a')<<nl;



}

int main(){
    Fast_IO();
int t =1;
cin>>t;
while(t--){
solve();
}
return 0;
}