#include<bits/stdc++.h>
using namespace std;int st[35],k,n,nr,i,j;double tr[35][35],v[35],sum,sol,dp[35][35],comb; main(){freopen("dezastru.in","r",stdin);freopen("dezastru.out","w",stdout);scanf("%d%d",&n,&k);for(i=1;i<=n;i++)scanf("%lf",&v[i]);for(i=0;i<=n;i++)dp[i][0]=1;for(i=1;i<=n;i++)for(int j=1;j<=i;j++)dp[i][j]=dp[i-1][j]+dp[i-1][j-1]*v[i];for(i=0;i<=n;i++)tr[i][0]=1.0;for(i=1;i<=(n);i++)for(j=1;j<=i;j++)tr[i][j]=tr[i-1][j]+tr[i-1][j-1];comb=tr[n][k];sol=(dp[n][k])/(comb);printf("%.6lf\n",sol);}