#include <bits/stdc++.h>

using namespace std;
ifstream in("dezastru.in");
ofstream out("dezastru.out");
double p[28],dp[26][26],c[60][26];
//suma prod. din primele x el. cu j factori
int main()
{  int n,k;
   in>>n>>k;
   for(int i=1;i<=n;i++)
    in>>p[i];
    for(int i=0;i<=n;i++)
        dp[i][0]=c[i][0]=1;
   for(int i=1;i<=n;i++)
    for(int j=1;j<=k;j++)
    dp[i][j]=p[i]*dp[i-1][j-1]+dp[i-1][j];
    //1.adaugam el. i 2.precedent
    for(int i=1;i<=n;i++)
      for(int j=1;j<=k;j++)
       c[i][j]=c[i-1][j]+c[i-1][j-1];
   out<<setprecision(6)<<fixed<<dp[n][k]/c[n][k];
    return 0;
}