#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, i;
  string s;
  cin >> n >> k;
  if (k > n || (n != 1 && k == 1)) {
    printf("-1\n");
    return 0;
  }
  s.push_back('a');
  char p = 'b';
  for (i = 1; i <= n - 1; i++) {
    if (n - i == k - 2) {
      while (k - 2 != 0) {
        p++;
        k--;
        s.push_back(p);
      }
      break;
    } else {
      if (i % 2 == 0) {
        s.push_back('a');
      } else
        s.push_back('b');
    }
  }
  cout << s << endl;
  return 0;
}
