#include<bits/stdc++.h>
using namespace std;
#define fastIO ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define int long long int
#define fi first
#define se second
#define pub push_back
#define pi pair<int,int>
#define all(v) (v).begin(), (v).end()
#define rep(i, l, r) for(int i=(int)(l);i<(int)(r);i++)
#define repd(i, l, r) for (int i=(int)(l);i>=(int)(r);i--)
#define clrg(i, l, r) for(int i=(int)(l);i<(int)(r);i++)vis[i]=0,v[i].clear();
int power(int x, unsigned int y){int res = 1;while (y > 0){ if (y & 1){res = res*x;} y = y>>1;x = x*x;}return res;}
int powermod(int x, unsigned int y, int p){int res = 1;x = x % p;while (y > 0){if (y & 1){res = (res*x) % p;}y = y>>1; x = (x*x) % p;}return res;}
#define print2d(mat,n,m){for(int i=0;i<(int)(n);i++){for(int j=0;j<(m);j++){cout<<mat[i][j]<<" ";}cout<< endl;}}
#define clr(a,x) memset(a,x,sizeof(a))
#define rr(v) for(auto &val:v)
#define print(v) for (const auto itr : v){cout<<itr<<' ';}cout<<"\n";
#define ln length()
#define sz size()
#define mod 1000000007
#define elif else if
int ans,ans1=0,ans2=0;
 int solve(string s,char ch){
    //cout<<ch<<"\n";
   int n=s.ln,mn1=0,mn2=0,a1=0,a2=0;
   if(n==1){
      //ans1+=(s[0]!=ch);
      //ans1+=(s[0]!=ch);
      return (s[0]!=ch);
   }
   rep(i,0,n/2){
      mn1+=(s[i]!=ch);
   }
   rep(i,n/2,n){
      mn2+=(s[i]!=ch);
   }
   //cout<<s<<"\n";
   //cout<<mn1<<" "<<mn2<<"\n";
      a1+=mn1;
      a1+=solve(s.substr(n/2,n/2),char(ch+1));
      a2+=mn2;
      a2+=solve(s.substr(0,n/2),char(ch+1));
      //cout<<a1<<" "<<a2<<" "<<ch<<"\n";
      return min(a1,a2);
}
int32_t main(){
        fastIO
   int T;
   cin>>T;
   while(T--){
      int n;
      cin>>n;
      string s;
      cin>>s;
      ans=1e18;
      ans1=0,ans2=0;
      ans=solve(s,'a');
      cout<<ans<<"\n";
   }
   return 0;
}