#include <bits/stdc++.h>
#define nmax 28

using namespace std;

ifstream fin ("dezastru.in");
ofstream fout ("dezastru.out");

int n,k;
double v[nmax],d[nmax][nmax];

double fact(int q){
    if(!q)return 1;
    return fact(q-1)*q;
}

int main()
{
    fin>>n>>k;
    for(int i=1;i<=n;++i)
        fin>>v[i];
    for(int i=0;i<=n;++i)d[i][0]=1;
    for(int i=1;i<=n;++i)
        for(int j=1;j<=i;++j)
            d[i][j]=d[i-1][j]+d[i-1][j-1]*v[i];
    fout<<setprecision(6)<<fixed<<d[n][k]*fact(k)*fact(n-k)/fact(n);
    return 0;
}