#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int n, k;
  cin >> n >> k;
  if (k == 1 && n == 1) {
    cout << "a";
    return 0;
  }
  if (k > n) {
    cout << -1;
    return 0;
  }
  if (k == 1) {
    cout << -1;
    return 0;
  }
  for (int i = 1; i <= n - k + 2; i++) {
    if (i % 2 == 1)
      cout << "a";
    else
      cout << "b";
  }
  for (int i = 99; i <= 98 + k - 2; i++) {
    char g;
    g = i;
    cout << g;
  }
}
