#include "math.h"
#include <algorithm>
#include <complex>
#include <cstdio>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <string>
#include <vector>
#define rep(i, n) for (lli i = 0; i < (n); i++)
#define rrep(i, n) for (lli i = (n)-1; i >= 0; i--)
using namespace std;
typedef long long int lli;
int main()
{
    int n, m;
    cin >> n >> m;
    int b, a[100005];
    cin >> b;
    rep(i, n - 1)
    {
        cin >> a[i];
    }
    sort(a, a + n - 1);
    int u = n;
    int l = 0;
    // m個高い
    while (u - l > 1) {
        int mid = (u + l) / 2;
        //cout << mid << endl;
        int score = (b + a[mid]);
        // socreよりを多い組をmこ作れる可
        int high = n - 2;
        int low = 0;
        int temp = m;
        while (temp && high > low) {
            if (low == mid)
                low++;
            if (high == mid) {
                high--;
            }
            while (a[high] + a[low] <= score) {
                low++;
                if (low == mid)
                    low++;
                if (high <= low) {
                    break;
                }
            }
            temp--;
            high--;
            low++;
        }
        if (temp == 0) {
            l = mid;
        } else {
            u = mid;
        }
    }
    if (u == n)
        u = -1;
    cout << u << endl;
}