#include<bits/stdc++.h>
#define int long long
#define all(v)v.begin(),v.end()
#define rall(v)v.rbegin(),v.rend()
bool st;
using namespace std;
template<typename T>istream&operator>>(istream&I,vector<T>&v){for(auto&i:v)I>>i;return I;}
template<typename T>ostream&operator<<(ostream&O,vector<T> v){for(auto i:v)O<<i<<' ';return O;}
string TMP,STR;
stringstream CIN;
void read_all(){
#ifdef debug
	cerr<<"----------------------------------- START -----------------------------------\n";
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	while(getline(cin,TMP))STR+=TMP,STR+='\n';
	CIN.str(STR);
	cerr<<"\n--------------------------------- INPUT END ---------------------------------\n";
#endif
}
#ifdef debug
#define cin CIN
#endif
namespace AC{
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)y=min(y,c[npos]+f(t+1,npos,1));
	}
	return res=min(x,y);
}
void solve(){
	cin>>n>>m;
	for(int i=1;i<=n-2;++i)cin>>c[i];
	for(int i=0;i<T;i++)for(int j=0;j<=n;j++)for(int k=0;k<2;k++)dp[i][j][k]=-2;
	P=1.0/m;
	double ans=f(0,0,0);
	cout<<setprecision(20)<<ans<<endl;
}
}
bool ed;
signed main(){
	chrono::steady_clock::time_point St=chrono::steady_clock::now();
	read_all();
	chrono::steady_clock::time_point ST=chrono::steady_clock::now();
	int t=1;
	//cin>>t;
	while(t--)AC::solve();
	chrono::steady_clock::time_point ED=chrono::steady_clock::now();
	cerr<<setprecision(3)<<fixed<<"\n\n------------------------------------ END ------------------------------------\n"<<"Total Time   : "<<left<<setw(10)<<chrono::duration_cast<chrono::milliseconds>(ED-St).count()<<" \tms\n"<<"Real  Time   : "<<left<<setw(10)<<chrono::duration_cast<chrono::milliseconds>(ED-ST).count()<<" \tms\n"<<"             : "<<left<<setw(10)<<chrono::duration_cast<chrono::microseconds>(ED-ST).count()<<" \t?s\n\n"<<"Total Memory : "<<left<<setw(10)<<(&ed-&st)/1048576.0<<" \tMB\n"<<"               "<<left<<setw(10)<<(&ed-&st)/1024.0<<" \tKB\n"<<"               "<<left<<setw(10)<<(&ed-&st)<<" \tByte\n";
}