#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main(){
    int N, K;
    cin >> N >> K;
    vector<int> v;
    for(int i = 0; i < N; i++){
        int a;
        cin >> a;
        v.push_back(a);
    }
    sort(v.begin(), v.end());
    double Max, Min;
    double ans;
    
    if(K % 2 == 0 || K == 3 || K == N || N >= K + 2){
        Max = v[N-1];
        Min = v[0];
    }else if(K % 2 == 1 && K != 3){
        Max = v[N-1];
        Min = (v[0] + v[1]) / 2;
    }
    ans = Max - Min;
    cout << ans << endl;
    return 0;
}