#include<bits/stdc++.h>
using namespace std;
const int N=5e5+7;
int n; 
double dp[N],p[N];
int hd,tl;
struct pl{
	int l,r,p;
}q[N];
double calc(int x,int y){
	return p[n-y]*dp[y]+(1-p[n-y])*(n-x);
}
signed main(){
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout<<fixed<<setprecision(20);
	cin>>n;
	for(int i=1;i<=n;i++)
		cin>>p[i];
	dp[0]=n;
	for(int i=0;i<n;i++){
		for(int j=i+1;j<=n;j++){
			dp[j]=max(dp[j],calc(j,i));
		}
	}
	q[hd=tl=1]={1,n,0};
	for(int i=1;i<=n;i++){
	    while(hd<tl&&q[hd].r<i)hd++;
		dp[i]=calc(i,q[hd].p);
	    while(hd<tl&&calc(q[tl].l,i)>=calc(q[tl].l,q[tl].p))tl--;
	    int L=q[tl].l,R=n,mid,ans=n+1;
	    while(L<=R)mid=L+R>>1,calc(mid,i)>=calc(mid,q[tl].p)?R=mid-1,ans=mid:(L=mid+1);
	    if(ans<=n)q[tl].r=ans-1,q[++tl]={ans,n,i};
	}
	cout<<dp[n]<<"\n";
	return 0;
}