#include <bits/stdc++.h>
int main() {
  int n, k;
  int i, j;
  char s[1000001];
  int ch;
  scanf("%d %d", &n, &k);
  if (n == 1 && k == 1) {
    printf("a\n");
    return 0;
  }
  if (n < k || k == 1) {
    printf("-1\n");
    return 0;
  }
  for (i = 0; i < n - (k - 2); i++) {
    s[i] = (i % 2 == 0) ? 'a' : 'b';
  }
  for (j = i, ch = 'c'; j < n; j++, ch++) {
    s[j] = (char)ch;
  }
  s[j] = '\0';
  printf("%s\n", s);
  return 0;
}
