#include<cstring>
#include<cmath>
#include<iostream>
#include<algorithm>
#include<cstdio>
using namespace std;
typedef long long ll;
const int maxm=1e5+7;
int n;
ll a[maxm],b[maxm];
ll c1[maxm];
ll c2[maxm];
ll c3[maxm];
int l;
double ans=0;
int lowbit(int x)
{
return x&(-x);
}
void add1(int x,ll y)
{
for(;x;x-=lowbit(x))
c1[x]=max(c1[x],y);
}
void add2(int x,ll y)
{
for(;x<=l;x+=lowbit(x))
c2[x]=max(c2[x],y);
}
void add3(int x,ll y)
{
for(;x;x-=lowbit(x))
c3[x]=max(c3[x],y);
}
ll ask1(int x)
{
ll ans=0;
for(;x<=l;x+=lowbit(x))
ans=max(ans,c1[x]);
return ans;
}
ll ask2(int x)
{
ll ans=0;
for(;x;x-=lowbit(x))
ans=max(ans,c2[x]);
return ans;
}
ll ask3(int x)
{
ll ans=0;
for(;x<=l;x+=lowbit(x))
ans=max(ans,c3[x]);
return ans;
}
int main()
{
scanf("%d",&n);
for(int i=1;i<=n;i++)
scanf("%lld",a+i),b[i]=a[i];
sort(b+1,b+n+1);
l=unique(b+1,b+n+1)-b-1;
for(int i=1;i<=n;i++)
{
a[i]=lower_bound(b+1,b+l+1,a[i])-b+1;//防止有0
}
l++;
for(int i=n;i>=1;i--)
{
ll w1=ask1(a[i]+1),w2=ask2(a[i]-1),w3=ask3(a[i]+1);
ans=max(ans,(1.0*w3+b[a[i]-1])/2.0);
ans=max(ans,(1.0*w2+b[a[i]-1])/2.0);
ans=max(ans,1.0*w1+b[a[i]-1]);
add1(a[i],b[a[i]-1]+w1);
add2(a[i],b[a[i]-1]+w2);
add3(a[i],b[a[i]-1]+max(w2,w3));
}
printf("%.3lf",ans);
return 0;
}