#include<bits/stdc++.h>
using namespace std;
using ll=long long;
const ll INF=(ll)4e18;
int main(){ios::sync_with_stdio(0);cin.tie(0);
 int N; if(!(cin>>N)) return 0;
 vector<ll>T(N),c(N,INF); priority_queue<pair<ll,int>,vector<pair<ll,int>>,greater<>>q;
 for(int i=0;i<N;i++){cin>>T[i]; c[i]=T[i]; q.push({c[i],i});}
 while(!q.empty()){
  auto [t,u]=q.top(); q.pop(); if(t!=c[u]) continue;
  for(int d:{-1,1}){
    int v=u+d; if(v<0||v>=N) continue;
    ll x=t+T[v]/2; if(x<c[v]){c[v]=x; q.push({x,v});}
    int other = v + (d==1?1:-1); // <- 여기서 '다른 쪽 이웃'을 정확히 계산
    if(other>=0 && other<N && c[other]<INF){
      x = max(t,c[other]) + T[v]/4;
      if(x<c[v]){c[v]=x; q.push({x,v});}
    }
  }
 }
 cout<<*max_element(c.begin(),c.end())<<"\n";
}

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