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