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