#include <iostream>
#include <bits/stdc++.h>
 
using namespace std;
 
int compute(int l, int r, string s, char c){
    if(l==r) return s[l] == c ? 0 : 1;
    int mid = l+(r-l)/2;
    int cnt1=0,cnt2=0;
    for(int i=l; i<=mid; i++){
        if(s[i] != c) cnt1++;
    }
    for(int i=mid+1; i<=r; i++){
        if(s[i] != c) cnt2++;
    }
    return min(compute(mid+1,r,s,c+1)+cnt1,compute(l,mid,s,c+1)+cnt2);
}
 
int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int t,n;
    string s;
    cin>>t;
    while(t-- > 0){
        cin>>n>>s;
        cout<<compute(0,n-1,s,'a')<<"\n";
    }
    return 0;
}

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Ngo4CmJiZGNhYWFhCjgKYXNkZmdoamsKOApjZWFhYWFiYgo4CmJiYWFkZGNjCjEKegoyCmFj

6
8
bbdcaaaa
8
asdfghjk
8
ceaaaabb
8
bbaaddcc
1
z
2
ac