#include <bits/stdc++.h>
int main() {
  int n, k, tmp;
  char ac;
  while (scanf("%d%d", &n, &k) == 2) {
    if (n < k) {
      printf("-1\n");
      continue;
    }
    if (n > 1 && k == 1) {
      printf("-1\n");
      continue;
    }
    if (n == 1 && k == 1) {
      printf("a\n");
      continue;
    }
    tmp = n - k + 2;
    for (int i = 0; i < tmp; i++) {
      if (i & 1)
        printf("b");
      else
        printf("a");
    }
    tmp = k - 2;
    ac = 'c';
    for (int i = 0; i < tmp; i++) {
      printf("%c", ac);
      ac++;
    }
    printf("\n");
  }
}
