1. //Utkarsh.25dec
  2. #include <bits/stdc++.h>
  3. #define ll long long int
  4. #define pb push_back
  5. #define mp make_pair
  6. #define mod 1000000007
  7. #define vl vector <ll>
  8. #define all(c) (c).begin(),(c).end()
  9. using namespace std;
  10. ll power(ll a,ll b) {ll res=1;a%=mod; assert(b>=0); for(;b;b>>=1){if(b&1)res=res*a%mod;a=a*a%mod;}return res;}
  11. ll modInverse(ll a){return power(a,mod-2);}
  12. const int N=500023;
  13. bool vis[N];
  14. vector <int> adj[N];
  15. long long readInt(long long l,long long r,char endd){
  16.     long long x=0;
  17.     int cnt=0;
  18.     int fi=-1;
  19.     bool is_neg=false;
  20.     while(true){
  21.         char g=getchar();
  22.         if(g=='-'){
  23.             assert(fi==-1);
  24.             is_neg=true;
  25.             continue;
  26.         }
  27.         if('0'<=g && g<='9'){
  28.             x*=10;
  29.             x+=g-'0';
  30.             if(cnt==0){
  31.                 fi=g-'0';
  32.             }
  33.             cnt++;
  34.             assert(fi!=0 || cnt==1);
  35.             assert(fi!=0 || is_neg==false);
  36.  
  37.             assert(!(cnt>19 || ( cnt==19 && fi>1) ));
  38.         } else if(g==endd){
  39.             if(is_neg){
  40.                 x= -x;
  41.             }
  42.  
  43.             if(!(l <= x && x <= r))
  44.             {
  45.                 cerr << l << ' ' << r << ' ' << x << '\n';
  46.                 assert(1 == 0);
  47.             }
  48.  
  49.             return x;
  50.         } else {
  51.             assert(false);
  52.         }
  53.     }
  54. }
  55. string readString(int l,int r,char endd){
  56.     string ret="";
  57.     int cnt=0;
  58.     while(true){
  59.         char g=getchar();
  60.         assert(g!=-1);
  61.         if(g==endd){
  62.             break;
  63.         }
  64.         cnt++;
  65.         ret+=g;
  66.     }
  67.     assert(l<=cnt && cnt<=r);
  68.     return ret;
  69. }
  70. long long readIntSp(long long l,long long r){
  71.     return readInt(l,r,' ');
  72. }
  73. long long readIntLn(long long l,long long r){
  74.     return readInt(l,r,'\n');
  75. }
  76. string readStringLn(int l,int r){
  77.     return readString(l,r,'\n');
  78. }
  79. string readStringSp(int l,int r){
  80.     return readString(l,r,' ');
  81. }
  82. void solve()
  83. {
  84.     int n=readInt(1,100000,' ');
  85.     int x=readInt(1,min(n,26),'\n');
  86.     if(n==1)
  87.     {
  88.         cout<<"a"<<'\n';
  89.         return;
  90.     }
  91.     if(x>((n+1)/2))
  92.     {
  93.         cout<<-1<<'\n';
  94.         return;
  95.     }
  96.     string tmp="";
  97.     for(int i=0;i<x;i++)
  98.         tmp+=('a'+i);
  99.     while(tmp.length()<n/2)
  100.         tmp+=('a'+x-1);
  101.     string rev="";
  102.     for(int i=0;i<n/2;i++)
  103.         rev+=tmp[i];
  104.     reverse(all(rev));
  105.     if(n%2==1 && tmp.length()<(n+1)/2)
  106.         tmp+='a';
  107.     tmp+=rev;
  108.     cout<<tmp<<'\n';
  109. }
  110. int main()
  111. {
  112.     #ifndef ONLINE_JUDGE
  113.     freopen("input.txt", "r", stdin);
  114.     freopen("output.txt", "w", stdout);
  115.     #endif
  116.     ios_base::sync_with_stdio(false);
  117.     cin.tie(NULL),cout.tie(NULL);
  118.     int T=readInt(1,10000,'\n');
  119.     while(T--)
  120.         solve();
  121.     assert(getchar()==-1);
  122.     cerr << "Time : " << 1000 * ((double)clock()) / (double)CLOCKS_PER_SEC << "ms\n";
  123. }