/*
 * zyrch - Rupanshu Yadav
 */
#ifdef CREED 
#include "creed.h"
#else
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <fstream>
#include <queue>
#include <deque>
#include <bitset>
#include <iterator>
#include <list>
#include <stack>
#include <map>
#include <set>
#include <functional>
#include <utility>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <iomanip>
#include <cassert>
#include <sstream>
#pragma GCC target ("avx2")
#pragma GCC optimization ("O3")
#pragma GCC optimization ("unroll-loops")
#endif
 
 
using namespace std;
 
typedef long long ll;
typedef long double ld;
typedef double db;
typedef string str;
typedef pair<int, int> pi;
typedef pair<ll,ll> pl;
typedef pair<ld,ld> pd;
 
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<ld> vd;
typedef vector<str> vs;
typedef vector<pi> vpi;
typedef vector<pl> vpl;
 
#define fast_cin() ios_base::sync_with_stdio(false); cin.tie(NULL) 
 
#define forn(i,a,b) for (int i = (a); i < ll(b); ++i)
#define fo(i,a) forn(i,0,ll(a))
#define rforn(i,a,b) for (int i = (b)-1; i >= (a); --i)
#define ro(i,a) rforn(i,0,a)
#define trav(a,x) for (auto& a: x)
 
#define sz(x) (int)x.size()
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
#define rsz resize
#define ins insert
 
#define mp make_pair
#define pb push_back
#define eb emplace_back
#define f first
#define s second
 
void __print(int x) {cerr << x;}
void __print(long x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x) {cerr << x;}
void __print(unsigned long long x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
 
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifdef CREED
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif
 
const ll INF = 9e18;
 
ll mod = 1000000007;
inline ll fast_expo(ll base,ll power,ll modulo=mod){
    base%=modulo;
    if (base<0) base+=modulo;
    ll x=base,cnt=power,ans=1;
    while(cnt){
        if (cnt&1) ans=(ans*x)%modulo;
        x=(x*x)%modulo;
        cnt>>=1;
    }
    return ans;
}
 
ll takemod(ll a, ll mo){
    a %= mo;
    if(a < 0) a+=mo;
    return a;
}
 
// if mod is prime
ll modinv(ll a, ll mo){
    return takemod(fast_expo(takemod(a,mo), mo-2, mo), mo);
}
// 297586BQ
//=========================CODE STARTS HERE=============================//

int main() {
    fast_cin();
    
    ll n, k;
    cin >> n >> k;
    
    // adding new edges give n - 2 more white vertices
    // start with one white
    // add longer first (exchange arugments)
    // add to the shortest depth available (greedy)
    
    // How to cal ans?
    // Segtree binary search on min prefix with atleast k white
    // binary search of ans(proof ?) and calculate possible for each answer
    
    vl arr(n);
    fo(i, n) cin >> arr[i];
    
    sort(all(arr), greater<ll>());
    
    ll lo = 0, hi = 2e5;
    
    while(lo < hi) {
	ll mid = lo + (hi - lo)/2;
	
	ll total_white = 0;
	ll current_depth = 0;
	ll pre_sum_on_depth = 1;
	ll current_depth_count = 1;
	ll wvertices_less_mid = 1;
	bool can = false;
	ll upper, lower;
	vl depth_count(2e5, 0);
	
	depth_count[0] = 1;
	depth_count[1] = -1;
	 
	fo(i, n) {
	    while (current_depth_count == 0) {
		current_depth++;
		pre_sum_on_depth += depth_count[current_depth];
		current_depth_count = pre_sum_on_depth;
	    }
	    
	    if (current_depth > mid) break;
	    
	    current_depth_count--;
	    wvertices_less_mid--;
	    
	    total_white += arr[i] - 2;
	    depth_count[current_depth + 1]--;
	    depth_count[current_depth + 1]++;
	    
	    lower = current_depth + 2;
	    upper = current_depth + 2 + (arr[i] - 3)/2;
	    
	    depth_count[lower] += 2;
	    depth_count[upper + 1] -= 2;
	    
	    wvertices_less_mid += (min(mid, upper) - lower + 1) * 2;
	    
	    if (arr[i] % 2 == 0) {
		depth_count[upper + 1]++;
		depth_count[upper + 2]--;
		if (upper + 1 <= mid)
		    wvertices_less_mid++;
	    }
	    // check if kth closest white vertex is at depth mid or greater
	    if (wvertices_less_mid >= k) {
		can = true;
		break;
	    }
	}
	
	if (can) {
	    hi = mid;
	}else {
	    lo = mid + 1;
	}
    }
    
    cout << (hi != 2e5 ? hi : -1) << '\n';
    
    return 0;
}
