#include <bits/stdc++.h>
using namespace std;
const int inf = 2147483647;
const int N = 1e6 + 100;
int getint();
char s[N];
int main() {
  int n = getint(), k = getint(), i;
  if (k > n || (k == 1 && n != 1)) {
    printf("-1");
    return 0;
  }
  while (k > 2) s[n--] = 96 + k--;
  for (i = 1; i <= n; i++) s[i] = i & 1 ? 'a' : 'b';
  puts(s + 1);
  return 0;
}
int getint() {
  int w = 0;
  bool q = 1;
  char c = getchar();
  while ((c < '0' || c > '9') && c != '-') c = getchar();
  if (c == '-') q = 0, c = getchar();
  while (c >= '0' && c <= '9') w = w * 10 + c - '0', c = getchar();
  return q ? w : -w;
}
