#include<fstream>

using namespace std;

const char iname[]="dezastru.in";
const char oname[]="dezastru.out";
const int maxn=26;

ifstream f(iname);
ofstream g(oname);

double a[maxn][maxn],p[maxn];

int n,i,j,c[maxn][maxn],k;

int main()
{
    f>>n>>k;
    c[0][0]=1;
    for(i=1;i<=n;++i)
    {
        c[i][0]=1;
        for(j=1;j<=i;++j)
            c[i][j]=c[i-1][j]+c[i-1][j-1];
    }
    for(i=1;i<=n;++i)
        f>>p[i];
    for(i=0;i<=n;++i)
        a[0][i]=1;
    for(i=1;i<=k;++i)
        for(j=1;j<=n;++j)
            a[i][j]=a[i][j-1]+a[i-1][j-1]*p[j];
    a[k][n]/=c[n][k];
    g.setf(ios::fixed,ios::floatfield);
    g.precision(6);
    g<<a[k][n]<<"\n";
}