#include <bits/stdc++.h>
using namespace std;
#pragma comment(linker, "/STACK:1024000000,1024000000")
const double pi = 2.0 * asin(1.0), eps = 1e-12;
const int maxn = 1100, maxm = 1100000, inf = 0x3f3f3f3f;
int main() {
  int i, j, n, m;
  while (scanf("%d%d", &n, &m) == 2) {
    if (n == m && n == 1) {
      cout << "a" << endl;
      continue;
    }
    if (n < m) {
      puts("-1");
    } else if (n > 1 && m == 1) {
      puts("-1");
    } else {
      for (i = 0; i < n - m + 2; i++) {
        if (i % 2 == 0)
          putchar('a');
        else
          putchar('b');
      }
      for (j = 2; j < m; j++) {
        putchar(j + 'a');
      }
      printf("\n");
    }
  }
  return 0;
}
