#include <bits/stdc++.h>
int dr[] = {2, 2, -2, -2, 1, -1, 1, -1};
int dc[] = {1, -1, 1, -1, 2, 2, -2, -2};
int dr1[] = {0, 0, 1, 1, 1, -1, -1, -1};
int dc1[] = {1, -1, 1, 0, -1, 0, 1, -1};
int dr2[] = {0, 0, 1, -1};
int dc2[] = {1, -1, 0, 0};
using namespace std;
int main() {
  long long int cn, n, k, i;
  while (cin >> n >> k) {
    char s[n + 5];
    if (n == k && n == 1)
      cout << "a" << endl;
    else if (k > n || (k == 1))
      cout << "-1" << endl;
    else {
      cn = n;
      for (i = k; i > 2; i--) {
        s[cn] = 'a' + i - 1;
        cn--;
      }
      for (i = 1; i <= cn; i++) {
        if (i % 2 == 1)
          s[i] = 'a';
        else
          s[i] = 'b';
      }
      s[n + 1] = '\0';
      for (i = 1; i <= n; i++) cout << s[i];
      cout << endl;
    }
  }
  return 0;
}
