#include <bits/stdc++.h>
int main() {
  int n, k, i, s = 0;
  char a[2] = {'a', 'b'}, b = 'c';
  scanf("%d %d", &n, &k);
  if (k > n) {
    printf("-1");
  } else if (n == 1 && k == 1)
    printf("a");
  else if (k == 1) {
    printf("-1");
  } else {
    for (i = 0; i < (n - (k - 2)); i++) {
      printf("%c", a[s]);
      s = !s;
    }
    for (i = (n - (k - 2)); i < n; i++) {
      printf("%c", b);
      b++;
      if (b - 'a' == 26) b = 'c';
    }
  }
  return 0;
}
