#include <bits/stdc++.h>

using namespace std;

typedef long double ld;

int n,k;

ld dp[30];
ld sum=0;

int main()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    cin>>n>>k;
    for(int i=1;i<=n;i++)
    {
        ld x;
        cin>>x;
        for(int cnt=min(i,k);cnt>=1;cnt--)
        {
            dp[cnt]=(dp[cnt-1]*x+dp[cnt]+(cnt==1)*x);
        }
    }
    for(ld x=k+1;x<=n;x++)
    {
        dp[k]/=x;
    }
    cout<<fixed<<setprecision(6)<<dp[k]<<"\n";
    return 0;
}