#include <fstream>
using namespace std;

ifstream cin( "dezastru.in" );
ofstream cout( "dezastru.out" );

double dp[ 40 ][ 40 ];
double s, x;
int n, t;

int main()
{
    cin >> n >> t;
    dp[ 0 ][ 0 ] = 1;
    for( int i = 1; i <= n; i++ )
        for( int j = dp[ i ][ 0 ] = 1; j <= i; j++ ) {
            cin >> x;
            dp[ i ][ j ] = dp[ i - 1 ][ j ] + x * dp[ i - 1 ][ j - 1 ];
        }
    
    s = dp[ n ][ t ];
    for( int i = t + 1; i <= n; ++i ) 
        s /= i;
    for( int i = 1; i <= n - t; i++ ) 
        s *= i;

    cout << s << '\n';
    return 0;
 }