#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  int a;
  int b, i, j;
  cin >> a >> b;
  char s[a];
  if (a < b || (b == 1 && a > 1))
    cout << -1;
  else {
    for (i = 0; i < a - (b - 2); i++) {
      if (i % 2 == 0)
        s[i] = 'a';
      else
        s[i] = 'b';
    }
    char m = 'c';
    for (j = i; j < a; j++) {
      s[j] = m++;
    }
    for (j = 0; j < a; j++) cout << s[j];
  }
  return 0;
}
