#include <bits/stdc++.h>
int main() {
  int n, k, i;
  scanf("%d%d", &n, &k);
  if (n == 1 && k == 1)
    printf("a");
  else if (n < k || k == 1)
    printf("-1\n");
  else {
    printf("ab");
    int dif = k - 2;
    n = n - 2;
    int rem = n - dif;
    for (i = 0; i < (rem) / 2; i++) printf("ab");
    if (rem % 2) printf("a");
    for (i = 2; dif; i++, dif--) printf("%c", 'a' + i);
  }
  return 0;
}
