description: Grandma Capa has decided to knit a scarf and asked Grandpa Sher to make a pattern for it, a pattern is a string consisting of lowercase English letters. Grandpa Sher wrote a string s of length n.
  
  Grandma Capa wants to knit a beautiful scarf, and in her opinion, a beautiful scarf can only be knit from a string that is a palindrome. She wants to change the pattern written by Grandpa Sher, but to avoid offending him, she will choose one lowercase English letter and erase some (at her choice, possibly none or all) occurrences of that letter in string s.
  
  She also wants to minimize the number of erased symbols from the pattern. Please help her and find the minimum number of symbols she can erase to make string s a palindrome, or tell her that it's impossible. Notice that she can only erase symbols equal to the one letter she chose.
  
  A string is a palindrome if it is the same from the left to the right and from the right to the left. For example, the strings 'kek', 'abacaba', 'r' and 'papicipap' are palindromes, while the strings 'abb' and 'iq' are not.
  
  Input
  
  The first line contains a single integer t (1 ≤ t ≤ 100) — the number of test cases. The next 2 ⋅ t lines contain the description of test cases. The description of each test case consists of two lines.
  
  The first line of each test case contains a single integer n (1 ≤ n ≤ 10^5) — the length of the string.
  
  The second line of each test case contains the string s consisting of n lowercase English letters.
  
  It is guaranteed that the sum of n over all test cases does not exceed 2 ⋅ 10^5.
  
  Output
  
  For each test case print the minimum number of erased symbols required to make the string a palindrome, if it is possible, and -1, if it is impossible.
  
  Example
  
  Input
  
  
  5
  8
  abcaacab
  6
  xyzxyz
  4
  abba
  8
  rprarlap
  10
  khyyhhyhky
  
  
  Output
  
  
  2
  -1
  0
  3
  2
  
  Note
  
  In the first test case, you can choose a letter 'a' and erase its first and last occurrences, you will get a string 'bcaacb', which is a palindrome. You can also choose a letter 'b' and erase all its occurrences, you will get a string 'acaaca', which is a palindrome as well.
  
  In the second test case, it can be shown that it is impossible to choose a letter and erase some of its occurrences to get a palindrome.
  
  In the third test case, you don't have to erase any symbols because the string is already a palindrome.
solution: import java.io.*;
  import java.util.*;
  
  
  
  import java.math.*;
  import java.math.BigInteger;
  
  
  public final class B
  {    
  	static PrintWriter out = new PrintWriter(System.out); 
  	static StringBuilder ans=new StringBuilder();
  	static FastReader in=new FastReader();
  	static ArrayList<Integer> g[];
  	static long mod=(long) 998244353,INF=Long.MAX_VALUE;
  	static boolean set[],col[]; 
  	static int par[],flip[];
  	static int D[],P[][];
  	static int sum=0,size[];
  	static int seg[];
  	static int A[],B[];
  	static long fact[],Xor[];
  	public static void main(String args[])throws IOException
  	{	
  		/*
  		 * star,rope
  		 */
  		int T=i();
  		outer:while(T-->0)
  		{
  			int N=i();
  			char X[]=in.next().toCharArray();
  			boolean f=false;
  			int min=-1;
  			xyz:for(int i=0; i<26; i++)
  			{
  				int l=0,r=N-1;
  				int c=0;
  				while(l<=r)
  				{
  					if(X[l]==X[r])
  					{
  						l++;
  						r--;
  					}
  					else if(X[l]-'a'==i) {
  						
  						c++;
  						l++;
  					}
  					else if(X[r]-'a'==i)
  					{
  						c++;
  						r--;
  					}
  					else continue xyz;
  				}
  				f=true;
  				if(min==-1)min=c;
  				min=Math.min(min,c);
  			}
  			ans.append(min+"\n");
  		
  		}
  		out.println(ans);
  		out.close();
  	}
  
  	static int ask(StringBuilder sb,int a)
  	{
  		System.out.println(sb+""+a);
  		return i();
  	}
  	static void swap(char X[],int i,int j)
  	{
  		char x=X[i];
  		X[i]=X[j];
  		X[j]=x;
  	}
  	static int min(int a,int b,int c)
  	{
  		return Math.min(Math.min(a, b), c);
  	}
  	static long nCr(int n,int c,int r)//log(N)*2 time
  	{
  		long s=fact[n];
  		long a=pow(fact[c],mod-2)%mod;
  		long b=pow(fact[r],mod-2)%mod;
  		a=(a*b)%mod;
  		s=(s*a)%mod;
  		return s;
  	}
  
  	static long and(int i,int j)
  	{
  		System.out.println("and "+i+" "+j);
  		return l();
  	}
  	static long or(int i,int j)
  	{
  		System.out.println("or "+i+" "+j);
  		return l();
  	}
  	static int len=0,number=0;
  	static void f(char X[],int i,int num,int l)
  	{
  		if(i==X.length)
  		{
  			if(num==0)return;
  			//update our num
  			if(isPrime(num))return;
  			if(l<len)
  			{
  				len=l;
  				number=num;				
  			}
  			return;
  		}
  		int a=X[i]-'0';
  		f(X,i+1,num*10+a,l+1);
  		f(X,i+1,num,l);
  	}
  	static boolean is_Sorted(int A[])
  	{
  		int N=A.length;
  		for(int i=1; i<=N; i++)if(A[i-1]!=i)return false;
  		return true;
  	}
  	static boolean f(StringBuilder sb,String Y,String order)
  	{
  		StringBuilder res=new StringBuilder(sb.toString());
  		HashSet<Character> set=new HashSet<>();
  		for(char ch:order.toCharArray())
  		{
  			set.add(ch);
  			for(int i=0; i<sb.length(); i++)
  			{
  				char x=sb.charAt(i);
  				if(set.contains(x))continue;
  				res.append(x);
  			}
  		}
  
  		String str=res.toString();
  		return str.equals(Y);
  	}
  	static boolean all_Zero(int f[])
  	{
  		for(int a:f)if(a!=0)return false;
  		return true;
  	}
  	static long form(int a,int l)
  	{
  		long x=0;
  		while(l-->0)
  		{
  			x*=10;
  			x+=a;
  		}
  		return x;
  	}
  	static int count(String X)
  	{
  		HashSet<Integer> set=new HashSet<>();
  		for(char x:X.toCharArray())set.add(x-'0');
  		return set.size();
  	}
  
  	static int f(long K)
  	{
  		long l=0,r=K;
  		while(r-l>1)
  		{
  			long m=(l+r)/2;
  			if(m*m<K)l=m;
  			else r=m;
  		}
  		return (int)l;
  	}
  	//	static void build(int v,int tl,int tr,long A[])
  	//	{
  	//		if(tl==tr)
  	//		{
  	//			seg[v]=A[tl];
  	//		}
  	//		else
  	//		{
  	//			int tm=(tl+tr)/2;
  	//			build(v*2,tl,tm,A);
  	//			build(v*2+1,tm+1,tr,A);
  	//			seg[v]=Math.min(seg[v*2], seg[v*2+1]);
  	//		}
  	//	}
  	static long ask(int v,int tl,int tr,int l,int r)
  	{
  		if(l>r)return (long)(1e10);
  		if(tl==l && tr==r)
  		{
  			return seg[v];
  		}
  		int tm=(tl+tr)/2;
  		return Math.min(ask(v*2,tl,tm,l,Math.min(tm, r)), ask(v*2+1,tm+1,tr,Math.max(l,tm+1),r));
  	}
  	static int [] sub(int A[],int B[])
  	{
  		int N=A.length;
  		int f[]=new int[N];
  		for(int i=N-1; i>=0; i--)
  		{
  			if(B[i]<A[i])
  			{
  				B[i]+=26;
  				B[i-1]-=1;
  			}
  			f[i]=B[i]-A[i];
  		}
  		for(int i=0; i<N; i++)
  		{
  			if(f[i]%2!=0)f[i+1]+=26;
  			f[i]/=2;
  		}
  		return f;
  	}
  	static int[] f(int  N)
  	{
  		char X[]=in.next().toCharArray();
  		int A[]=new int[N];
  		for(int i=0; i<N; i++)A[i]=X[i]-'a';
  		return A;
  	}
  
  
  
  
  	static int max(int a ,int b,int c,int d)
  	{
  		a=Math.max(a, b);
  		c=Math.max(c,d);
  		return Math.max(a, c);
  	}
  	static int min(int a ,int b,int c,int d)
  	{
  		a=Math.min(a, b);
  		c=Math.min(c,d);
  		return Math.min(a, c);
  	}
  
  
  
  	static HashMap<Integer,Integer> Hash(int A[])
  	{
  		HashMap<Integer,Integer> mp=new HashMap<>();
  		for(int a:A)
  		{
  			int f=mp.getOrDefault(a,0)+1;
  			mp.put(a, f);
  		}
  		return mp;
  	}
  	static long mul(long a, long b)
  	{
  		return ( a %mod * 1L * b%mod )%mod;
  	}
  	static void swap(int A[],int a,int b)
  	{
  		int t=A[a];
  		A[a]=A[b];
  		A[b]=t;
  	}
  
  
  	static int find(int a)
  	{
  		if(par[a]<0)return a;
  		return par[a]=find(par[a]);
  	}
  	static void union(int a,int b)
  	{
  		a=find(a);
  		b=find(b);
  		if(a!=b)
  		{
  			par[a]+=par[b];
  			par[b]=a;
  		}
  	}
  	static boolean isSorted(int A[])
  	{
  		for(int i=1; i<A.length; i++)
  		{
  			if(A[i]<A[i-1])return false;
  		}
  		return true;
  	}
  
  	static boolean isDivisible(StringBuilder X,int i,long num)
  	{
  		long r=0;
  		for(; i<X.length(); i++)
  		{
  			r=r*10+(X.charAt(i)-'0');
  			r=r%num;
  		}
  		return r==0;
  	}    
  
  
  
  	static int lower_Bound(int A[],int low,int high, int x) 
  	{ 
  		if (low > high) 
  			if (x >= A[high]) 
  				return A[high]; 
  
  		int mid = (low + high) / 2; 
  
  		if (A[mid] == x) 
  			return A[mid]; 
  
  		if (mid > 0 && A[mid - 1] <= x && x < A[mid]) 
  			return A[mid - 1]; 
  
  		if (x < A[mid]) 
  			return lower_Bound( A, low, mid - 1, x); 
  
  		return lower_Bound(A, mid + 1, high, x); 
  	} 
  
  	static String f(String A)
  	{
  		String X="";
  		for(int i=A.length()-1; i>=0; i--)
  		{
  			int c=A.charAt(i)-'0';
  			X+=(c+1)%2;
  		}
  		return X;
  	}
  
  	static void sort(long[] a) //check for long
  	{
  		ArrayList<Long> l=new ArrayList<>();
  		for (long i:a) l.add(i);
  		Collections.sort(l);
  		for (int i=0; i<a.length; i++) a[i]=l.get(i);
  	}
  
  	static String swap(String X,int i,int j)
  	{
  		char ch[]=X.toCharArray();
  		char a=ch[i];
  		ch[i]=ch[j];
  		ch[j]=a;
  		return new String(ch);
  	}
  
  	static int sD(long n)  
  	{  
  
  		if (n % 2 == 0 )  
  			return 2;  
  
  		for (int i = 3; i * i <= n; i += 2) {  
  			if (n % i == 0 )  
  				return i;  
  		}  
  
  		return (int)n;  
  	}  
  
  	static void setGraph(int N)
  	{
  		flip=new int[N+1];
  		par=new int[N+1];
  		D=new int[N+1];
  		g=new ArrayList[N+1];
  		for(int i=0; i<=N; i++)
  		{
  
  			g[i]=new ArrayList<>();		
  			D[i]=Integer.MAX_VALUE;
  		}
  	}
  
  
  
  	static  long pow(long a,long b)
  	{
  		//long mod=1000000007;
  		long pow=1;
  		long x=a;
  		while(b!=0)
  		{
  			if((b&1)!=0)pow=(pow*x)%mod;
  			x=(x*x)%mod;
  			b/=2;
  		}
  		return pow;
  	}
  
  	static long toggleBits(long x)//one's complement || Toggle bits
  	{
  		int n=(int)(Math.floor(Math.log(x)/Math.log(2)))+1;
  
  		return ((1<<n)-1)^x;
  	}
  
  	static int countBits(long a)
  	{
  		return (int)(Math.log(a)/Math.log(2)+1);
  	}
  
  	static long fact(long N)
  	{ 
  		long n=2; 
  		if(N<=1)return 1;
  		else
  		{
  			for(int i=3; i<=N; i++)n=(n*i)%mod;
  		}
  		return n;
  	}
  
  	static int kadane(int A[])
  	{
  		int lsum=A[0],gsum=A[0];
  		for(int i=1; i<A.length; i++)
  		{
  			lsum=Math.max(lsum+A[i],A[i]);
  			gsum=Math.max(gsum,lsum);
  		}
  		return gsum;
  	}
  
  	static void sort(int[] a) {
  		ArrayList<Integer> l=new ArrayList<>();
  		for (int i:a) l.add(i);
  		Collections.sort(l);
  		for (int i=0; i<a.length; i++) a[i]=l.get(i);
  	}
  
  	static boolean isPrime(long N)
  	{
  		if (N<=1)  return false; 
  		if (N<=3)  return true; 
  		if (N%2 == 0 || N%3 == 0) return false; 
  		for (int i=5; i*i<=N; i=i+6) 
  			if (N%i == 0 || N%(i+2) == 0) 
  				return false; 
  		return true; 
  	}
  	static void print(char A[])
  	{
  		for(char c:A)System.out.print(c+" ");
  		System.out.println();
  	}
  	static void print(boolean A[])
  	{
  		for(boolean c:A)System.out.print(c+" ");
  		System.out.println();
  	}
  	static void print(int A[])
  	{
  		for(int a:A)System.out.print(a+" ");
  		System.out.println();	
  	}
  	static void print(long A[])
  	{
  		for(long i:A)System.out.print(i+ " ");
  		System.out.println();
  
  	}
  	static void print(boolean A[][])
  	{
  		for(boolean a[]:A)print(a);
  	}
  	static void print(long A[][])
  	{
  		for(long a[]:A)print(a);
  	}
  	static void print(int A[][])
  	{
  		for(int a[]:A)print(a);
  	}
  	static void print(ArrayList<Integer> A)
  	{
  		for(int a:A)System.out.print(a+" ");
  		System.out.println();
  	}
  
  	static int i()
  	{
  		return in.nextInt();
  	}
  
  	static long l()
  	{
  		return in.nextLong();
  	}
  
  	static int[] input(int N){
  		int A[]=new int[N];
  		for(int i=0; i<N; i++)
  		{
  			A[i]=in.nextInt();
  		}
  		return A;
  	}
  
  	static long[] inputLong(int N)     {
  		long A[]=new long[N];
  		for(int i=0; i<A.length; i++)A[i]=in.nextLong();
  		return A;
  	}
  
  	static long GCD(long a,long b) 
  	{
  		if(b==0)
  		{
  			return a;
  		}
  		else return GCD(b,a%b );
  	}
  
  }
  
  
  
  
  //Code For FastReader
  //Code For FastReader
  //Code For FastReader
  //Code For FastReader
  class FastReader
  {
  	BufferedReader br;
  	StringTokenizer st;
  	public FastReader()
  	{
  		br=new BufferedReader(new InputStreamReader(System.in));
  	}
  
  	String next()
  	{
  		while(st==null || !st.hasMoreElements())
  		{
  			try
  			{
  				st=new StringTokenizer(br.readLine());
  			}
  			catch(IOException e)
  			{
  				e.printStackTrace();
  			}
  		}
  		return st.nextToken();
  	}
  
  	int nextInt()
  	{
  		return Integer.parseInt(next());
  	}
  
  	long nextLong()
  	{
  		return Long.parseLong(next());
  	}
  
  	double nextDouble()
  	{
  		return Double.parseDouble(next());
  	}
  
  	String nextLine()
  	{
  		String str="";
  		try
  		{
  			str=br.readLine();
  		}
  		catch (IOException e)
  		{
  			e.printStackTrace();
  		}
  		return str;
  	}
  
  }