#include <bits/stdc++.h>
using namespace std;
int GCD(int a, int b) {
  if (!a) return b;
  return GCD(b % a, a);
}
string s;
int main() {
  int n, k;
  cin >> n >> k;
  if (n < k || (n > 1 && k == 1))
    puts("-1");
  else {
    if (n == 1 && k == 1) {
      puts("a");
      return 0;
    }
    int i = 0, p = 0;
    while (i + k - 1 <= n) {
      switch (p) {
        case 0:
          s += "a";
          break;
        case 1:
          s += "b";
          break;
      }
      p = (p + 1) % 2;
      i++;
    }
    for (int j = 'c'; j <= 'z' && i < n; j++, i++) {
      s += j;
    }
    cout << s;
  }
  return 0;
}
