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

#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
#define pb push_back
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

int main() {
	cin.tie(0)->sync_with_stdio(0);
    int n, k;
    cin >> n >> k;
    vector<ll> a(n);
    for(int i = 0; i < n; i++)
        cin >> a[i];
    sort(all(a));
    ll sum = 0;
    ll sq = 0;
    long double best = 1e18;
    for(int i = 0; i < n; i++) {
        sq += a[i]*a[i];
        sum += a[i];
        if(i >= k) {
            sq -= a[i-k]*a[i-k];
            sum -= a[i-k];
        }
        if(i >= k-1) {
            long double mean = (long double)sum/k;
            best = min(best, k*mean*mean-2*mean*sum+sq);
        }
    }
    cout << fixed << setprecision(10) << best << "\n";
}