#include <fstream>
#include <iomanip>
using namespace std;
double dp[26][26],p[26];
int comb[26][26];
int main()
{   int n,k,i,j;double sol;
    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)
        dp[i][0]=1;
    for (i=1;i<=n;++i)
    for (j=1;j<=i;++j)
      dp[i][j]=dp[i-1][j]+dp[i-1][j-1]*p[i];

    comb[0][0]=comb[1][0]=comb[1][1]=1;

    for (i=2;i<=n;++i)
    for (j=1;j<=i;++j)
      comb[i][j]=comb[i-1][j-1]+comb[i-1][j];

    sol=dp[n][k]/double(comb[n][k]);
    g<<setprecision(6)<<sol;
    return 0;
}