#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(){std::ifstream f("dezastru.in");std::ofstream g("dezastru.out");f>>n>>k;FOR(i,n)f>>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);g<<std::fixed<<std::setprecision(6)<<sol;}