#include <bits/stdc++.h>
using namespace std;
int main() {
  int N, M;
  cin >> N >> M;
  if (N == 1 && M == 1) {
    cout << "a";
  } else if (N < M || (M == 1))
    cout << -1 << endl;
  else {
    char c = 'b';
    for (int i = 0; i < N - M + 2; i++) {
      if (i % 2 == 0)
        cout << "a";
      else
        cout << "b";
    }
    c = c + 1;
    for (int i = N - M + 2; i < N; i++) {
      cout << c;
      c = c + 1;
    }
  }
}
