#include<bits/stdc++.h>
#define FOR(i,n) for(i=1;i<=n;i++)
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,n)scanf("%lf",&v[i]);for(i=0;i<=n;i++)dp[i][0]=1;FOR(i,n)FOR(j,i) 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,n)FOR(j,i)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);}