#include <bits/stdc++.h>
using namespace std;
int n, k;
vector<char> s;
int main() {
  cin >> n >> k;
  if (k > n) {
    cout << -1;
    cin >> n;
    return 0;
  }
  if (k == 1 && n > 1) {
    cout << -1;
    cin >> n;
    return 0;
  }
  for (int i = 0; i < n; i++) {
    if (i % 2 == 0) {
      s.push_back('a');
    } else {
      s.push_back('b');
    }
  }
  int x = 98 + k - 2;
  for (int i = n - 1; i >= n - k + 2; i--) {
    s[i] = (char)x;
    x--;
  }
  for (int i = 0; i < n; i++) {
    cout << s[i];
  }
  cin >> n;
  return 0;
}
