#include<bits/stdc++.h>
using namespace std;

int main()
{
	int t;
	cin>>t;
	while(t--){
	    int n,x,i;
	    cin>>n>>x;
	    if(n==1&&x==1){
	        cout<<"a"<<endl;
	    }else if(n%2==0){
	        if(x!=n/2){
	            cout<<"-1"<<endl;
	        }else{
	            for(i=1;i<=n/2;i++){
	                cout<<((char)(i+96));
	            }
	            for(i=(n/2);i>=1;i--){
	                cout<<((char)(i+96));
	            }
	            cout<<endl;
	        }
	    }else if(n%2==1){
	        if(x!=n/2+1){
	            cout<<"-1"<<endl;
	        }
	        else{
	            for(i=1;i<=n/2;i++){
	                cout<<((char)(i+96));
	            }
	            for(i=(n/2)+1;i>=1;i--){
	                cout<<((char)(i+96));
	            }
	            cout<<endl;
	        }
	    }
	}
	return 0;
}