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