package yukicoder;

import java.util.*;

public class Q463 {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int m = sc.nextInt();
		double[] c = new double[n];
		for (int i = 1; i < n - 1; ++i) {
			c[i] = sc.nextDouble();
		}

		// 魔法を使わずにマスi(0<=i<n-1)からn-1まで行くのに必要な回数の期待値
		double[] dp1 = new double[n];

		// A(Evec+a)=Evec
		// (A-E)x=-Aa
		double[][] mx = new double[n - 1][n - 1];
		for (int i = 0; i < n - 1; ++i) {
			double p = 1.0 / m;
			for (int j = 1; j <= m; ++j) {
				if (i + j < n - 1) {
					mx[i][i + j] += p;
				} else if (i + j == n - 1) {
					continue;
				} else {
					int pos = (n - 1) - ((i + j) - (n - 1));
					mx[i][pos] += p;
				}
			}
		}
		double[] v = new double[n - 1];
		for (int i = 0; i < n - 1; ++i) {
			v[i] = -c[i];
		}
		v = mul(mx, v);
		for (int i = 0; i < n - 1; ++i) {
			mx[i][i] -= 1;
		}

		// [mx]x=vをxについて解きたい.
		for (int i = 0; i < n - 1; ++i) {
			int pos = i;
			while (pos < n - 1 && mx[pos][i] == 0) {
				++pos;
			}
			if (pos == n - 1)
				continue;
			double[] tmp = mx[i];
			mx[i] = mx[pos];
			mx[pos] = tmp;
			double tmp3 = v[i];
			v[i] = v[pos];
			v[pos] = tmp3;
			double tmp2 = mx[i][i];
			for (int j = 0; j < n - 1; ++j) {
				mx[i][j] /= tmp2;
			}
			v[i] /= tmp2;

			for (int j = 0; j < n - 1; ++j) {
				if (mx[j][i] == 0)
					continue;
				if (j == i)
					continue;
				double tmp4 = mx[j][i];
				for (int k = 0; k < n - 1; ++k) {
					mx[j][k] -= mx[i][k] * tmp4;
				}
				v[j] -= v[i] * tmp4;
			}
		}
		double[] e = new double[n];
		Arrays.fill(e, Double.MAX_VALUE);
		e[n - 1] = 0;
		outer: for (int i = n - 2; i >= 0; --i) {
			double e1 = 0;
			for (int j = m; j >= 1; --j) {
				if (i + j >= n - 1) {
					e[i] = 0;
					continue outer;
				} else {
					e1 += 1.0 / m * (e[i + j] + c[i + j]);
				}
			}
			double e2 = 1L << 60;
			for (int j = m; j >= 1; --j) {
				e2 = Math.min(e2, v[i + j] + c[i + j]);
			}
			e[i] = Math.min(e1, e2);
		}
		System.out.printf("%.20f",e[0]);
	}

	static void show(double[][] mx) {
		int n = mx.length;
		for (int i = 0; i < n; ++i) {
			for (int j = 0; j < n; ++j) {
				System.out.print(mx[i][j] + " ");
			}
			System.out.println();
		}
	}

	static double[] mul(double[][] a, double[] v) {
		int n = a.length;
		double[] ret = new double[n];
		for (int i = 0; i < n; ++i) {
			for (int j = 0; j < n; ++j) {
				ret[i] += a[i][j] * v[j];
			}
		}
		return ret;

	}

	static void tr(Object... objects) {
		System.out.println(Arrays.deepToString(objects));
	}
}