import java.util.*;
import java.io.*;
//import java.math.*;

public class Task{
        //  ..............code begins here..............

static long mod=(long)998244353;
static int res=998244353;
static String ss="abcdefghijklmnopqrstuvwxyz";
static void rec(char[] c, int c1,int i,int j,int tmp){
	char cc=ss.charAt(c1);
	if(i==j){
		if(c[i]!=cc) tmp++;
		res=Math.min(res,tmp);
		return;
	}
	int k=(i+j)/2,tc=0,tc1=0;
	for(int x=i;x<=k;x++) if(c[x]!=cc) tc++;
	for(int x=k+1;x<=j;x++) if(c[x]!=cc) tc1++;
	rec(c,c1+1,i,k,tc1+tmp);rec(c,c1+1,k+1,j,tmp+tc);
}
static void solve() throws  IOException {
			int n=int_v(read()); res=998244353;
			char[] c=read().toCharArray();
			rec(c,0,0,n-1,0);
			out.write(res+"\n");
}
public static void main(String[] args) throws  IOException{
                assign();
                int t=int_v(read());
                while(t--!=0) solve();
                out.flush();
}

// taking inputs
static long pow1(long a,int p){long res=1;while(p>0){if((p&1)!=0){res=(res*a);}p >>=1;a=(a*a);}return res;}

static BufferedReader s1;
static BufferedWriter out;
static String read() throws IOException{String line="";while(line.length()==0){line=s1.readLine();continue;}return line;}
static int int_v (String s1){return Integer.parseInt(s1);}
static long long_v(String s1){return Long.parseLong(s1);}
static void sort(int[] a){List<Integer> l=new ArrayList<>();for(int x:a){l.add(x);}Collections.sort(l);for(int i=0;i<a.length;i++){a[i]=l.get(i);}}
static int[] int_arr() throws IOException{String[] a=read().split(" ");int[] b=new int[a.length];for(int i=0;i<a.length;i++){b[i]=int_v(a[i]);}return b;}
static long[] long_arr() throws IOException{String[] a=read().split(" ");long[] b=new long[a.length];for(int i=0;i<a.length;i++){b[i]=long_v(a[i]);}return b;}
static void assign(){s1=new BufferedReader(new InputStreamReader(System.in));out=new BufferedWriter(new OutputStreamWriter(System.out));}
//static String setpreciosion(double d,int k){BigDecimal d1 = new BigDecimal(Double.toString(d));return d1.setScale(k,RoundingMode.HALF_UP).toString();}//UP DOWN HALF_UP
static int gcd(int a,int b){if(b==0){return a;}return gcd(b,a%b);}
static long Modpow(long a,long p,long m){long res=1;while(p>0){if((p&1)!=0){res=(res*a)%m;}p >>=1;a=(a*a)%m;}return res%m;}
static long Modmul(long a,long b,long m){return ((a%m)*(b%m))%m;}
static long ModInv(long a,long m){return Modpow(a,m-2,m);}
//static long nck(int n,int r,long m){if(r>n){return 0l;}return Modmul(f[n],ModInv(Modmul(f[n-r],f[r],m),m),m);}
//static long[] f;
}
