#include "bits/stdc++.h"
using namespace std;
#define FOR(i,j,k) for(int (i)=(j);(i)<(int)(k);++(i))
#define rep(i,j) FOR(i,0,j)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define mt make_tuple
#define all(x) (x).begin(),(x).end()
#define debug(x) cout<<#x<<": "<<(x)<<endl
#define smax(x,y) (x)=max((x),(y))
#define smin(x,y) (x)=min((x),(y))
#define MEM(x,y) memset((x),(y),sizeof (x))
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<ll> vll;

const int T = 1001, N = 101;
int n, m;
double dp[T][N][2], c[100], P;

double f(int t, int pos, int mag) {
    double &res = dp[t][pos][mag];
    if(res > -1)return res;
    if(pos == n - 1 || t == T - 1) {
        return res = 0;
    }

    double x = 0, y = numeric_limits<double>::max();;

    for(int i = 1; i <= m; ++i) {
        int npos = pos + i;
        if(npos >= n)npos = 2 * (n - 1) - npos;
        x += P*(c[npos] + f(t + 1, npos, mag));
        if(!mag)smin(y, c[npos] + f(t + 1, npos, 1));
    }
    res = min(x, y);

    return res;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> n >> m;
    for(int i = 1; i <= n - 2; ++i)cin >> c[i];

    rep(i, T)rep(j, n + 1)rep(k, 2)dp[i][j][k] = -2;
    
    P = 1.0 / m;
    double ans = f(0, 0, 0);
    cout << setprecision(20) << ans << endl;
}