#include <bits/stdc++.h>
using namespace std;
template <typename T>
using V = vector<T>;
template <typename T>
using VV = vector<V<T>>;
template <typename T>
istream &operator>>(istream &cin, V<T> &arr) {
  for (T &val : arr) {
    cin >> val;
  }
  return cin;
}
template <typename T>
ostream &operator<<(ostream &cout, VV<T> &arr) {
  for (V<T> &val : arr) {
    cout << val << endl;
  }
  return cout;
}
template <typename T>
ostream &operator<<(ostream &cout, V<T> &arr) {
  for (T &val : arr) {
    cout << val << " ";
  }
  return cout;
}
void update(vector<vector<long long>> &tree, long long v, long long l,
            long long r, long long lb, long long rb, long long val) {
  if (lb > rb) {
    return;
  }
  if (l == lb && r == rb) {
    tree[v][2] += val;
  } else {
    long long m = (l + r) / 2;
    update(tree, v * 2, l, m, lb, min(rb, m), val);
    update(tree, v * 2 + 1, m + 1, r, max(lb, m + 1), rb, val);
  }
}
void build(vector<vector<long long>> &tree, vector<long long> &arr, long long v,
           long long l, long long r) {
  if (l == r)
    tree[v] = {arr[l], arr[l]};
  else {
    long long m = (l + r) / 2;
    build(tree, arr, v * 2, l, m);
    build(tree, arr, v * 2 + 1, m + 1, r);
    tree[v][0] = max(tree[v * 2][0], tree[v * 2 + 1][0]);
    tree[v][1] = min(tree[v * 2][1], tree[v * 2 + 1][1]);
  }
}
long long node(long long a, long long b) {
  while (b > 0) {
    a %= b;
    swap(a, b);
  }
  return a;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long n, need;
  cin >> n >> need;
  vector<long long> arr(n);
  long long s = 0;
  for (int i = 0; i < n; i++) {
    cin >> arr[i];
    s += arr[i];
  }
  if (s < need) {
    cout << -1;
    return 0;
  }
  arr.push_back(0);
  sort(arr.rbegin(), arr.rend());
  long long l = 1, r = n;
  while (l < r) {
    long long m = (l + r) / 2, sum = 0;
    for (long long i = 0, cnt = 0; i < n; cnt++) {
      long long temp = m;
      while (i < n && temp--) {
        sum += max(0LL, arr[i] - cnt);
        i++;
      }
    }
    if (sum < need)
      l = m + 1;
    else
      r = m;
  }
  cout << r;
  return 0;
}
