#include <bits/stdc++.h>
using namespace std;
long n, k;
long ans[1000010];
int main() {
  cin >> n >> k;
  if ((k > n) || ((k <= n) && (k == 1) && (n != 1))) {
    cout << "-1" << endl;
    return 0;
  }
  if ((n == 1) && (k == 1)) {
    cout << "a" << endl;
    return 0;
  }
  if (!(n & 1)) {
    ans[1] = 97;
    ans[2] = 98;
    long c(96 + k);
    for (long i = n; i >= n - (k - 2) + 1; --i) ans[i] = c--;
    cout << "ab";
    for (long i = 1; i <= ((n - k) / 2); ++i) cout << "ab";
    if (k & 1) cout << "a";
    for (long i = (n - (k - 2) + 1); i <= n; ++i) cout << char(ans[i]);
    cout << endl;
  } else {
    ans[1] = 97;
    ans[2] = 98;
    long c(96 + k);
    for (long i = n; i >= n - (k - 2) + 1; --i) ans[i] = c--;
    cout << "ab";
    for (long i = 1; i <= ((n - k) / 2); ++i) cout << "ab";
    if (!(k & 1)) cout << "a";
    for (long i = (n - (k - 2) + 1); i <= n; ++i) cout << char(ans[i]);
    cout << endl;
  }
  return 0;
}
