7. Субботник

import java.util.Scanner;

public class Main {
    static long[] a;
    static int n;
    static int r;
    static int c;

    static boolean isSumAllowed(long curMaxDiff) {
        boolean success;

        int groupsFound = 0;
        int start = 0;
        int end = 0;
        int count = 1;

        while (end < n - 1) {
            long curDiff = a[end + 1] - a[start];
            if (curDiff <= curMaxDiff) {
                end++;
                count++;
            } else while (a[end + 1] - a[start] > curMaxDiff) {
                start++;
                count--;
            }

            if (count >= c) {
                groupsFound++;
                start = ++end;
                count = 1;
            }

            if (groupsFound == r)
                break;
        }

        success = groupsFound >= r;
        return success;
    }

    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        n = in.nextInt();
        r = in.nextInt();
        c = in.nextInt();
        a = new long[n];

        for (int i = 0; i < n; i++) a[i] = in.nextLong();

        if (c == 1) {
            System.out.println(0);
            return;
        }

        Arrays.sort(a);

        long left = 0;
        long right = a[n - 1] - a[0];

        while (left < right) {
            long m = (left + right) / 2;
            boolean curSumAllowed = isSumAllowed(m);
            if (curSumAllowed)
                right = m;
            else
                left = m + 1;
        }
        System.out.print(right);
    }