#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  char ch = 'b';
  cin >> n >> k;
  int l = n - k;
  if (l < 0) {
    cout << -1 << endl;
    return 0;
  }
  if (k == 1) {
    if (n == 1)
      cout << 'a' << endl;
    else
      cout << -1 << endl;
    return 0;
  }
  int count = 0;
  for (int i = 0; i < l + 2; i++) {
    if (ch == 'a') {
      cout << 'b';
      ch = 'b';
    } else {
      cout << 'a';
      ch = 'a';
    }
  }
  ch = 'c';
  for (int i = l + 2; i < n; i++) {
    cout << ch;
    ch++;
  }
  cout << endl;
  return 0;
}
