#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;

typedef int _loop_int;
#define REP(i,n) for(_loop_int i=0;i<(_loop_int)(n);++i)
#define FOR(i,a,b) for(_loop_int i=(_loop_int)(a);i<(_loop_int)(b);++i)
#define FORR(i,a,b) for(_loop_int i=(_loop_int)(b)-1;i>=(_loop_int)(a);--i)

#define DEBUG(x) cout<<#x<<": "<<x<<endl
#define DEBUG_VEC(v) cout<<#v<<":";REP(i,v.size())cout<<" "<<v[i];cout<<endl
#define ALL(a) (a).begin(),(a).end()

#define CHMIN(a,b) a=min((a),(b))
#define CHMAX(a,b) a=max((a),(b))

// mod
const ll MOD = 1000000007ll;
#define FIX(a) ((a)%MOD+MOD)%MOD

// floating
typedef double Real;
const Real EPS = 1e-11;
#define EQ0(x) (abs(x)<EPS)
#define EQ(a,b) (abs(a-b)<EPS)
typedef complex<Real> P;

int n,m;
int a[125252];
int my;

bool ok(int you){
  int we = my + a[you];
  multiset<int> S;
  REP(i,n)if(i!=you)S.insert(a[i]);
  REP(i,m){
    auto it = S.end();it--;
    int mx = *it;S.erase(it);
    int rest = we - mx + 1;
    it = S.lower_bound(rest);
    if(it == S.end())return true;
    else{
      S.erase(it);
    }
  }
  return false;
}

int main(){
  scanf("%d%d",&n,&m);
  REP(i,n)scanf("%d",a+i);
  my = a[0];
  a[0] = a[n-1]; n--;
  sort(a,a+n);
  if(m-1 == n/2){
    cout<<a[0]<<endl;
    return 0;
  }
  // minimize
  int low = -1, high = n;
  while(low+1 < high){
    int mid = (low+high)/2;
    if(ok(mid))high = mid;
    else low = mid;
  }
  if(high==n){
    puts("-1");
  }else{
    cout<<a[high]<<endl;
  }
  return 0;
}