#include <bits/stdc++.h>
#define ll long long int
#define ld long double
using namespace std;

void mainSolve()
{
  int n, x;
  cin >> n >> x;
  if (n == 1)
  {
    cout << "a\n";
    return;
  }
  if (n < (2 * x) - 1)
  {
    cout << "-1\n";
    return;
  }
  string ans = "";
  int i = 0;
  for (char a = 'a'; i < x; i++, a++)
    ans += a;
  string last = ans;
  reverse(last.begin(), last.end());
  n -= (2 * x);
  if (n < 0)
  {
    cout << ans;
    for (int i = x - 2; i >= 0; i--)
      cout << ans[i];
    cout << endl;
    return;
  }
  while (n--)
    ans += 'a';
  ans += last;
  cout << ans << endl;
}

int main()
{
#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
  int t;
  cin >> t;
  while (t--)
  {
    mainSolve();
  }
  return 0;
}