import java.io.*;
import java.util.*;


public class Main_yukicoder463 {

	private static Scanner sc;
	private static Printer pr;

	private static void solve() {
		int n = sc.nextInt();
		int m = sc.nextInt();

		int[] a = new int[n];
		for (int i = 0; i < n - 2; i++) {
			a[i + 1] = sc.nextInt();
		}

		int lim = 1_000;

		double[][] ex = new double[lim][n];

		for (int k = 1; k < lim; k++) {
			for (int i = n - 2; i >= 0; i--) {
				for (int j = 1; j <= m; j++) {
					int tmp = i + j;
					if (i + j > n - 1) {
						tmp = n - 1 - (tmp - (n - 1));
					}

					ex[k][i] += (a[tmp] + ex[k - 1][tmp]) / m;
				}
			}
		}

//		pr.printf("%.10f\n", ex[lim - 1][0]);

		final double INF = Double.MAX_VALUE;
		double[] dp = new double[n];
		Arrays.fill(dp, INF);
		for (int i = n - 2; i >= 0; i--) {
			double dtmp = 0;
//			for (int j = 1; j <= m && i + j < n; j++) {
			for (int j = 1; j <= m; j++) {
				int tmp = i + j;
				if (i + j > n - 1) {
					tmp = n - 1 - (tmp - (n - 1));
				} else {
					dp[i] = Math.min(dp[i], ex[lim - 1][i + j] + a[i + j]);
				}
				dtmp += (a[tmp] + dp[tmp]) / m;
			}
			dp[i] = Math.min(dp[i], dtmp);
		}

		pr.printf("%.10f\n", dp[0]);
	}

	// ---------------------------------------------------
	public static void main(String[] args) {
		sc = new Scanner(System.in);
		pr = new Printer(System.out);

		solve();

		pr.close();
		sc.close();
	}

	private static class Printer extends PrintWriter {
		Printer(PrintStream out) {
			super(out);
		}
	}
}