#include<bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(int i=a;i<b;i++)



#define rrep(i,a,b) for(int i=a;i>=b;i--)
int N, M;
int C[101];
//-----------------------------------------------------------------
int main() {
	cin >> N >> M;
	rep(i, 2, N) cin >> C[i];

	vector<double> dp(N + 1, 0);
	rep(t, 0, 10000) {
		vector<double> pd(N + 1, 0);

		rep(i, 1, N) {
			pd[i] = C[i];
			rep(j, 1, M + 1) {
				int ii = i + j;
				if (ii <= N) pd[i] += dp[ii] / M;
				else pd[i] += dp[N - (ii - N)] / M;
			}
		}

		swap(dp, pd);
	}

	vector<double> dp2(N + 1, 0);
	rrep(i, N, 1) {
		if (N - i <= M) dp2[i] = C[i];
		else {
			double use = 1e20;
			rep(j, 1, M + 1) use = min(use, dp[i + j]);

			double nonuse = 0;
			rep(j, 1, M + 1) nonuse += dp2[i + j] / M;

			dp2[i] = C[i] + min(use, nonuse);
		}
	}

	printf("%.10f\n", dp2[1]);
}