#include <bits/stdc++.h>
using namespace std;
char s1[1000005];
int main() {
  int n, k;
  char ss[30] = ("abcdefghijklmnopqrstuvwxyz");
  while (cin >> n >> k) {
    if (n < k || k == 0 || k == 1 && n > 1) {
      cout << -1 << endl;
      continue;
    }
    int remain_char = k, i;
    memset(s1, '\0', sizeof(s1));
    for (i = 0; n - i != remain_char; ++i) {
      s1[i] = ss[i & 1];
      if (i < 2) --remain_char;
    }
    int base = 0;
    if (i < 2)
      base = i;
    else
      base = 2;
    strncat(s1, &ss[base], remain_char);
    cout << s1 << endl;
  }
  return 0;
}
