#include <iostream>
#include <fstream>
#include <iomanip>
#define nmax 100
using namespace std;
long double a[nmax][nmax];
long double p[nmax];
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");

int main()
{int i,n,k,j;
fin>>n>>k;
for(i=1;i<=n;i++)
fin>>p[i];
for(i=0;i<=n;i++)
a[i][0]=1;

    for(i=1;i<=n;i++)
        for(j=1;j<=i;j++)
            a[i][j]=a[i-1][j]+a[i-1][j-1]*p[i];


int imp=1;
for(i=n-k+1;i<=n;i++)imp*=i;
for(i=1;i<=k;i++)imp/=i;
    a[n][k]=a[n][k]/imp;
     fout<<setprecision(7)<<fixed<<a[n][k];
}