#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, i, j, f, d, ost;
  cin >> n >> m;
  if (m == 1 && n > 1) {
    cout << "-1";
    return 0;
  }
  if (m == 1 && n == 1) {
    cout << "a";
    return 0;
  }
  if (m > n) {
    cout << "-1";
    return 0;
  }
  char c = 'a';
  for (i = 0; i < n - m + 2; i++) {
    cout << c;
    if (c == 'a')
      c = 'b';
    else
      c = 'a';
  }
  c = 'c';
  for (i = n - m + 2; i < n; i++) {
    cout << c;
    c++;
  }
  return 0;
}
