#include <bits/stdc++.h>
using namespace std;
int main() {
  int n = 0, k = 0, i = 0;
  cin >> n >> k;
  if (n > 1 && k == 1) {
    cout << -1 << endl;
    return 0;
  }
  if (n == 1 && k == 1) {
    cout << "a";
    return 0;
  }
  if (k > n) {
    cout << -1 << endl;
    return 0;
  } else {
    for (i = 1; i <= n - k + 2; i++) {
      if (i % 2 == 1)
        cout << "a";
      else
        cout << "b";
    }
    int v = 99;
    for (i = 1; i <= k - 2; i++) {
      cout << (char)v;
      v++;
    }
  }
  return 0;
}
