#include <bits/stdc++.h>
using namespace std;
int m, n, t, k;
int main() {
  while (scanf("%d%d", &n, &k) == 2) {
    if (k > n || (n >= 2 && k == 1))
      printf("-1\n");
    else if (k == 1)
      printf("%c\n", 'a');
    else {
      int m = n - (k - 2);
      for (int i = 0; i < m; i++) printf("%c", i % 2 == 0 ? 'a' : 'b');
      for (int i = 0; i < k - 2; i++) printf("%c", 'a' + 2 + i);
      printf("\n");
    }
  }
  return 0;
}
