#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  if (k == 1) {
    if (n == 1)
      cout << 'a';
    else
      cout << -1;
    return 0;
  }
  if (k > n) {
    cout << -1;
    return 0;
  }
  string s = "s";
  for (int i = 1; i <= n; i += 2) {
    s += "ab";
  }
  for (int i = 3; i <= k; i++) {
    s[n - k + i] = 'a' - 1 + i;
  }
  for (int i = 1; i <= n; i++) {
    cout << s[i];
  }
  return 0;
}
