content: package v115;
  import java.io.BufferedReader;
  import java.io.FileReader;
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.InputStreamReader;
  import java.io.PrintWriter;
  import java.util.Arrays;
  import java.util.StringTokenizer;
  
  
  public class FewestFlops_UVa11552 {
  	
  	static final int INF = (int)1e9;
  	static int[] chunks, f[], memo[];
  	static int N;
  	
  	static int dp(int c, int idx)
  	{
  		if(idx == N)
  			return 0;
  		if(memo[c][idx] != -1)
  			return memo[c][idx];
  		int ret = INF, minus = f[c][idx] != 0 ? 1 : 0, chs = chunks[idx];
  		for(int i = 0; i < 26; ++i)
  			if(f[i][idx] != 0)
  				ret = Math.min(ret, chs - (chs > 1 && i == c || chs == 1 && i != c ? 0 : minus) + dp(i, idx + 1));
  		return memo[c][idx] = ret;
  	}
  	
  	public static void main(String[] args) throws IOException 
  	{
  		Scanner sc = new Scanner(System.in);
  		PrintWriter out = new PrintWriter(System.out);
  
  		int tc = sc.nextInt();
  		while(tc-->0)
  		{
  			 int k = sc.nextInt();
  			 String s = sc.next();
  			 N = s.length() / k;
  			 f = new int[27][N];
  			 for(int i = 0; i < N; ++i)
  				 for(int j = 0; j < k; ++j)
  					 f[s.charAt(i * k + j)-'a'][i]++;
  			 chunks = new int[N];
  			 for(int i = 0; i < N; ++i)
  			 {
  				 int count = 0;
  				 for(int j = 0; j < 26; ++j)
  					 if(f[j][i] != 0)
  						 ++count;
  				 chunks[i] = count;
  			 }
  			 
  			 memo = new int[27][N];
  			 for(int i = 0; i < 27; ++i)
  				 Arrays.fill(memo[i], -1);
  			 out.println(dp(26, 0));
  		}
  		out.flush();
  		out.close();
  	}
  
  	static class Scanner 
  	{
  		StringTokenizer st;
  		BufferedReader br;
  
  		public Scanner(InputStream s){    br = new BufferedReader(new InputStreamReader(s));}
  
  		public Scanner(FileReader r){    br = new BufferedReader(r);}
  
  		public String next() throws IOException 
  		{
  			while (st == null || !st.hasMoreTokens()) 
  				st = new StringTokenizer(br.readLine());
  			return st.nextToken();
  		}
  
  		public int nextInt() throws IOException {return Integer.parseInt(next());}
  
  		public long nextLong() throws IOException {return Long.parseLong(next());}
  
  		public String nextLine() throws IOException {return br.readLine();}
  
  		public double nextDouble() throws IOException
  		{
  			String x = next();
  			StringBuilder sb = new StringBuilder("0");
  			double res = 0, f = 1;
  			boolean dec = false, neg = false;
  			int start = 0;
  			if(x.charAt(0) == '-')
  			{
  				neg = true;
  				start++;
  			}
  			for(int i = start; i < x.length(); i++)
  				if(x.charAt(i) == '.')
  				{
  					res = Long.parseLong(sb.toString());
  					sb = new StringBuilder("0");
  					dec = true;
  				}
  				else
  				{
  					sb.append(x.charAt(i));
  					if(dec)
  						f *= 10;
  				}
  			res += Long.parseLong(sb.toString()) / f;
  			return res * (neg?-1:1);
  		}
  
  		public boolean ready() throws IOException {return br.ready();}
  
  
  	}
  }