#include <bits/stdc++.h>
using namespace std;
char s[1000005];
int main() {
  int n, k;
  while (scanf("%d%d", &n, &k) != EOF) {
    if (k > n || (n > 1 && k == 1)) {
      printf("-1\n");
      break;
    } else {
      if ((n - k) % 2) {
        for (int i = 1; i <= n - k + 1; i++) {
          s[i] = 'a';
          s[i + 1] = 'b';
          i++;
        }
        s[n - k + 2] = 'a';
        int c = 99;
        for (int i = n - k + 3; i <= n; i++) {
          s[i] = c;
          c++;
        }
      } else {
        for (int i = 1; i <= n - k; i++) {
          s[i] = 'a';
          s[i + 1] = 'b';
          i++;
        }
        int c = 97;
        for (int i = n - k + 1; i <= n; i++) {
          s[i] = c;
          c++;
        }
      }
    }
    for (int i = 1; i <= n; i++) printf("%c", s[i]);
    cout << endl;
  }
  return 0;
}
