#include <bits/stdc++.h>
using namespace std;
using ld = long double;

int main()
{
	cout << fixed << setprecision(10);
	int n, m;
	cin >> n >> m;
	vector<int> c(n);
	for (int i = 1; i < n - 1; i++) {
		cin >> c[i];
	}
	vector<ld> dp(n);
	for (int i = n - 1; i >= 0; i--) {
		if (n - 1 <= i + m) {
			dp[i] = c[i];
		}
		else {
			for (int j = 1; j <= m; j++) {
				dp[i] += (dp[i + j] + c[i]) / m;
			}
		}
	}
	cout << dp[0] << endl;
	return 0;
}