#include<bits/stdc++.h>
#include<unordered_set>
#include<unordered_map>
using namespace std;

#define MAX 100002

int n;
int m;

vector<int> v;

vector<int> vv;

multiset<int> s;
bool ok(int val){
	s.clear();
	for (int i = 0; i < vv.size(); i++){
		s.insert(vv[i]);
	}
	auto it = s.lower_bound(val);
	s.erase(it);
	int rankk = 0;
	while (!s.empty()){
		auto f = *s.rbegin();
		s.erase(s.lower_bound(f));
		int need = val + v[0] - f;
		need++;
		auto it = s.lower_bound(need);
		if (it != s.end()){
			s.erase(it);
			rankk++;
		}
		else{
			break;
		}
	}
	rankk++;
	return rankk <= m;
}
int main(){
	cin >> n >> m;
	for (int i = 0; i < n; i++){
		int a;
		scanf("%d", &a);
		v.push_back(a);
		if (i)vv.push_back(a);
	}
	sort(vv.begin(), vv.end());
	int mint = 0;
	int maxt = vv.size() - 1;
	while (mint + 1 < maxt){
		int mid = (mint + maxt) >> 1;
		if (ok(vv[mid])){
			maxt = mid;
		}
		else{
			mint = mid;
		}
	}
	if (ok(vv[mint])){
		cout << vv[mint] << endl;
	}
	else{
		if (ok(vv[maxt])){
			cout << vv[maxt] << endl;
		}
		else{
			puts("-1");
		}
	}
	return 0;
}