#include <fstream>
using namespace std;

ifstream in("dezastru.in");
ofstream out("dezastru.out");
int j,n,t,i;
double s,x,dp[40][40];
int main()
{
    in>>n>>t;
    dp[0][0]=1;
    for(i=1; i<=n; ++i)
    {in>>x;
        for(j=1; j<=i; ++j)
        {
            dp[i][0]=1;
            dp[i][j]=dp[i-1][j]+x*dp[i-1][j-1];
        }
    }
    s=dp[n][t];
    for(i=t+1; i<=n; ++i)
    {
        s/=i;
    }
    for(i=1; i<=n-t; ++i)
    {
        s*=i;
    }
    out<<s;
    return 0;
}