Help me in solving QTOO_2523 problem

My issue

answer is same but after submission it is giving wrong answer

My code

#include <bits/stdc++.h>
using namespace std;
 int alpha(int* a,int& n){
      for(int i=0;i<26;i++){
	        if(a[i]==2){
	            return n-2;
	        }
	        if(a[i]==n){
	            return 0;
	        }
      }
      return -1;
 }
int main() {
	int t,n;
	cin>>t;
	while(t--){
	    cin>>n;
	    string s;
	    cin>>s;
	    int a[26]={0};
	    for(int i=0;i<n;i++){
	        a[s[i]-'a']++;
	    }
	   cout<<alpha(a,n);
	   cout<<endl;
	   }

}

Problem Link: Bi_lindrome! Practice Coding Problem