#include <bits/stdc++.h>
using namespace std;
int i, j, k, x, y, z, n, m;
char c;
int main() {
  scanf("%d%d", &n, &m);
  if (m > n) {
    printf("-1\n");
    return 0;
  }
  if (m == 1) {
    if (n == 1)
      printf("a\n");
    else
      printf("-1\n");
    return 0;
  }
  c = '$';
  for (i = 1; i <= n - (m - 2); ++i)
    if (i & 1)
      printf("a");
    else
      printf("b");
  for (i = 3; i <= m; ++i) printf("%c", (char)(i + 'a' - 1));
  printf("\n");
  return 0;
}
