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