#include <bits/stdc++.h>
using namespace std;
int main() {
  int i, j, n, k;
  cin >> n >> k;
  char str[n];
  if (k == 1 && n == 1)
    cout << 'a';
  else if (n < k || (k == 1 && n > 1))
    cout << "-1";
  else {
    i = 0;
    while (i < n - k + 2) {
      str[i] = 'a';
      i = i + 2;
    }
    i = 1;
    while (i < n - k + 2) {
      str[i] = 'b';
      i = i + 2;
    }
    int p = 99;
    for (i = n - k + 2; i < n; i++) {
      str[i] = char(p);
      p++;
    }
    for (i = 0; i < n; i++) cout << str[i];
  }
  return 0;
}
