#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long int n, k;
  cin >> n >> k;
  if (k > n)
    cout << -1 << "\n";
  else if (n != 1 && k == 1)
    cout << -1 << "\n";
  else if (k == n) {
    char x = 'a';
    for (long long int i = 1; i <= (long long int)n; i++) {
      cout << x;
      x++;
    }
  } else if (k < n) {
    long long int t = n - (k - 2);
    for (long long int i = 1; i <= (long long int)t; i++) {
      if (i % 2 != 0)
        cout << "a";
      else
        cout << "b";
    }
    long long int q = n - t;
    char y = 'c';
    if (q > 0) {
      for (long long int i = 1; i <= (long long int)q; i++) {
        cout << y;
        y++;
      }
    }
  }
}
int main() {
  std::ios::sync_with_stdio(false);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
