#include <bits/stdc++.h>
using namespace std;
bool secdes(const pair<long long, long long>& a,
            const pair<long long, long long>& b) {
  if (a.second == b.second) {
    return a.first > b.first;
  } else {
    return a.second > b.second;
  }
}
bool sir(const pair<long long, long long>& a,
         const pair<long long, long long>& b) {
  if (a.first == b.first) {
    return a.second > b.second;
  } else {
    return a.first > b.first;
  }
}
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long n, k;
  cin >> n >> k;
  if (n < k)
    return cout << "-1", 0;
  else {
    if (n == 1)
      cout << "a";
    else if (n == 2) {
      if (k == 1)
        return cout << "-1", 0;
      else
        return cout << "ab", 0;
    } else {
      if (k == 1)
        return cout << "-1", 0;
      else {
        long long f = 1;
        for (long long i = 0; i <= n - k + 1; i++) {
          if (f == 1)
            cout << "a";
          else if (f == 2)
            cout << "b";
          if (f == 1)
            f = 2;
          else
            f = 1;
        }
        for (long long i = 2; i < k; i++) cout << (char)(i + 97);
      }
    }
  }
}
