C++ :

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

int n, k, a[100005], l, r, ans;

bool check(int x) {
    int now = 1, cnt = 0;
    for (int i = n; i >= now; i--) {
    	if (i == now) {
    		cnt++;
    		continue;
		}
        if (a[i] + a[now] <= x)
            now++;
        cnt++;
    }
    return cnt <= k;
}

int main() {
    cin >> n >> k;
    for (int i = 1; i <= n; i++) {
        cin >> a[i];
    }
    sort(a+1,a+1+n);
    l = a[n], r = a[n - 1] + a[n];
    while (l <= r) {
        int mid = (l + r) >> 1;
        if (check(mid)) 
            r = mid - 1, ans = mid;
        else l = mid + 1;
    }
    cout << ans;
    return 0;
}