#include <bits/stdc++.h>
#pragma comment(linker, "/stack:64000000")
using namespace std;
template <typename _T>
_T Max(_T a, _T b) {
  return a < b ? b : a;
}
template <typename _T>
_T Max(_T a, _T b, _T c) {
  return a < Max(b, c) ? Max(b, c) : a;
}
template <typename _T>
_T Min(_T a, _T b) {
  return a > b ? b : a;
}
template <typename _T>
_T Min(_T a, _T b, _T c) {
  return a > Min(b, c) ? Min(b, c) : a;
}
template <typename _T>
_T Abs(_T a) {
  return (a > 0) ? (a) : (-a);
}
const int inf = -(1 << 29);
const int INF = (1 << 29);
const int M = 105;
int mat[M * M];
int main() {
  int len, dis;
  while (cin >> len >> dis) {
    if (dis > len) {
      cout << -1 << endl;
    } else if (len == 1) {
      cout << "a" << endl;
    } else if (dis == 1) {
      cout << "-1" << endl;
    } else {
      int ab = len - dis + 2;
      for (int i = 0; i < ab / 2; i++) {
        cout << "ab";
      }
      (ab & 1) ? (cout << "a") : (cout << "");
      for (int i = 0; i < len - ab; i++) {
        printf("%c", 'c' + i);
      }
      cout << endl;
    }
  }
  return 0;
}
