#include<fstream>
using namespace std;
int n,k;
double a[30][30];
int c[30][30];
double p[30];
int i,j;
int main()
{
    ifstream f("dezastru.in");
    ofstream g("dezastru.out");
    f>>n>>k;
    for ( i = 1; i <= n; i++ )
        f>>p[i];

    for ( i = 0; i <= n; i++ )
        a[i][0] = 1;
    a[1][1] = p[1];
    for ( i = 2; i <= n; i++ )
        for ( j = 1; j <= i; j++ )
            a[i][j] = a[i-1][j] + a[i-1][j-1]*p[i];

    for ( i = 0; i <= n; i++ )
        c[i][0] = 1;
    for ( i = 1; i <= n; i++ )
        for ( j = 1; j <= i; j++ )
            c[i][j] = c[i-1][j] + c[i-1][j-1];

    g<<a[n][k]/(double)(c[n][k]);
    return 0;
}