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


public class Main_yukicoder463_1 {

	private static Scanner sc;
	private static Printer pr;

	private static void solve() {
		final long INF = -1;

		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();
		}

		if (m == n - 1) {
			pr.println(0);
			return;
		}

		long[][] ex = new long[n - 1][n];
		int[][] cnt = new int[n - 1][n];
		for (int i = 0; i < n - 1; i++) {
			Arrays.fill(ex[i], INF);
			ex[i][n - 1] = 0;
		}
		for (int i = 1; i <= m; i++) {
			ex[0][i] = a[i];
			cnt[0][i]++;
		}

//		double[] mm = new double[n - 1];
//		mm[0] = (double)1 / m;
//		for (int i = 1; i < n - 1; i++) {
//			mm[i] = mm[i - 1] / m;
//		}

		for (int k = 1; k < n - 1; k++) {
			for (int i = k; i < n - 1; i++) {
				if (i + m >= n - 1) {
					ex[k][n - 1] += Math.max(0, ex[k - 1][i]);
					cnt[k][n - 1] += cnt[k - 1][i];
				} else if (ex[k - 1][i] != INF) {
					for (int j = 1; j <= m; j++) {
						ex[k][i + j] = Math.max(0, ex[k][i + j]) + ex[k - 1][i] + a[i + j];
						cnt[k][i + j] += cnt[k - 1][i];
					}
				}
			}
		}

		double sum = 0;
		double mm = (double)1 / m;
		double sumcnt = 0;
		for (int i = 1; i < n - 1; i++) {
			sum += ex[i][n - 1] * mm;
			sumcnt += cnt[i][n - 1] * mm;
			mm /= m;
		}

		pr.printf("%.10f\n", sum);
//		pr.println(sumcnt);
	}

	// ---------------------------------------------------
	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);
		}
	}
}