#include <bits/stdc++.h>
using namespace std;
pair<int, int> operator+(const pair<int, int> &a, const pair<int, int> &b) {
  return make_pair(a.first + b.first, a.second + b.second);
}
pair<int, int> operator-(const pair<int, int> &a, const pair<int, int> &b) {
  return make_pair(a.first - b.first, a.second - b.second);
}
pair<int, int> &operator+=(pair<int, int> &a, const pair<int, int> &b) {
  a.first += b.first;
  a.second += b.second;
  return a;
}
pair<int, int> &operator-=(pair<int, int> &a, const pair<int, int> &b) {
  a.first -= b.first;
  a.second -= b.second;
  return a;
}
template <class T, class U>
bool cmp_second(const pair<T, U> &a, const pair<T, U> &b) {
  return a.second < b.second;
}
template <class T>
T gcd(T a, T b) {
  a = abs(a);
  b = abs(b);
  while (b) {
    T t = b;
    b = a % b;
    a = t;
  }
  return a;
}
template <class T>
pair<T, T> ext_gcd(T a, T b) {
  T a0 = 1, a1 = 0, b0 = 0, b1 = 1;
  if (a < 0) {
    a = -a;
    a0 = -1;
  }
  if (b < 0) {
    b = -b;
    b1 = -1;
  }
  while (b) {
    T t, q = a / b;
    t = b;
    b = a - b * q;
    a = t;
    t = b0;
    b0 = a0 - b0 * q;
    a0 = t;
    t = b1;
    b1 = a1 - b1 * q;
    a1 = t;
  }
  return make_pair(a0, a1);
}
inline int sg(int x) { return x ? (x > 0 ? 1 : -1) : 0; }
int n, k;
char s[1000500];
bool solve() {
  if (k > n) return 0;
  s[n] = '\0';
  if (k == 1) {
    if (n > 1)
      return 0;
    else
      s[0] = 'a';
  } else if (k == 2) {
    for (int i = 0; i < n; i++) s[i] = (i & 1) ? 'b' : 'a';
  } else {
    for (int i = 0; i < n; i++) s[i] = (i & 1) ? 'b' : 'a';
    for (int i = 2; i < k; i++) s[n - k + i] = 'a' + i;
  }
  puts(s);
  return 1;
}
int main(void) {
  scanf("%d %d", &(n), &(k));
  if (!solve()) puts("-1");
  return 0;
}
