#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <cstring>
#include <queue>
#include <set>
#include <array>
#include <algorithm>
#include <random>
#include <bitset>
#include "array"
#include <chrono>
//#include <ext/pb_ds/tree_policy.hpp>
//#include <ext/pb_ds/assoc_container.hpp>
using namespace std;
//using namespace __gnu_pbds;
//typedef tree<int, null_type, less<int>, rb_tree_tag,
// tree_order_statistics_node_update> pds;
#define all(x)x.begin(),x.end()
#define pack(x)sort(all(x));x.erase(unique(all(x)),x.end())
#define gi(x, v)lower_bound(all(x),v)-x.begin()
#define by(x) [](const auto& a, const auto& b) { return a.x < b.x; }
using ll = long long;
using ld = long double;
using tu = array<int, 2>;
#define int ll
template<class T>
struct erasePQ{
multiset<T> pq;
void push(T x){
pq.insert(x);
}
void erase(T x) {
if(!pq.count(x))return;
pq.erase(pq.find(x));
}
int size(){
return (int)pq.size();
}
T top(){
return *pq.rbegin();
}
T bot(){
return *pq.begin();
}
void clear(){
pq.clear();
}
};
erasePQ<int> mp;
erasePQ<int> now;
int arr[500'010];
int lft[500'010];
int rgt[500'010];
int last[500'010];
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
int n,a;
int ans=0;
cin>>n;
for(int i=1;i<=n;i++){
cin>>arr[i];
if(arr[i]!=-1)ans+=arr[i];
}
int idx=-1e9;
for(int i=1;i<=n;i++){
lft[i]=idx;
if(arr[i]==-1)idx=i;
}idx=1e9;
for(int i=n;i>=1;i--){
rgt[i]=idx;
if(arr[i]==-1)idx=i;
}
int add=1;
int pre=-1;
int sum = 0;
int addTime = -1;
int cnt=0;
for(int i=1;i<=n;i++){
addTime++;
if(arr[i]==-1){
cnt++;
if(pre==-1){
add--;
}else{
int tmp = now.top();
sum-=tmp;
mp.erase(tmp);
}
now.clear();
pre=-1;
add++;
continue;
}
if(arr[i]==0)continue;
int w = min(rgt[i]-i,i-lft[i])*2;
pre=max(pre,w);
last[w]=cnt;
while(arr[i]){
mp.push(w);
now.push(w);
sum+=w;
arr[i]--;
}
while(now.size()!=0 && addTime+sum+min(-w,-now.top()+rgt[i]-i -w/2)>(i-1)*2){
sum-=mp.top();
now.erase(mp.top());
mp.erase(mp.top());
}
}
{
if(pre==-1){
add--;
}else{
int tmp = now.top();
sum-=tmp;
mp.erase(tmp);
}
}
cout<<ans-mp.size()-add;
}
