#include <bits/stdc++.h>
using namespace std;
long long int m, n, i;
long long int a[1000000], ans, sum[1000000] = {0}, x;
int check(long long int k) {
  x = 0;
  for (i = 1; i <= n; i++) {
    if (a[i] - (i - 1) / k > 0) x = x + a[i] - (i - 1) / k;
  }
  if (x >= m)
    return 1;
  else
    return 0;
}
int binsearch(long long int low, long long int high) {
  while (low < high) {
    long long int mid = (low + high) / 2;
    if (check(mid))
      high = mid;
    else
      low = mid + 1;
  }
  if (!check(low)) low = -1;
  return low;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> m;
  for (i = 1; i <= n; i++) {
    cin >> a[i];
  }
  sort(a + 1, a + 1 + n, greater<int>());
  for (i = 1; i <= n; i++) sum[i] = sum[i - 1] + a[i];
  ans = binsearch(1, n);
  cout << ans;
  return 0;
}
