#include <bits/stdc++.h>
using namespace std;
int GCD(int x, int y) { return !y ? x : GCD(y, x % y); }
int LCM(int x, int y) { return x * y / GCD(x, y); }
long long ceilDivision(long long dividend, long long divisor) {
  return (dividend + divisor - 1) / divisor;
}
int main() {
  long long n, c;
  cin >> n >> c;
  if (n < c || (n > 1 && c < 2))
    cout << -1 << endl;
  else if (n == 1)
    cout << "a" << endl;
  else {
    for (long long i = 0; i < n - c + 2; i++)
      if (i % 2)
        cout << "b";
      else
        cout << "a";
    for (long long i = n + 2 - c, j = 1; i < n; j++, i++)
      cout << (char)(j + 'b');
    cout << endl;
  }
  return 0;
}
