//https://oj.uz/submission/680268
#include <bits/stdc++.h>
using namespace std;
#ifdef DEBUG
#define dbg(...) printf(__VA_ARGS__);
#define getchar_unlocked getchar
#else
#define dbg(...)
#endif
#define sf scanf
#define pf printf
#define fi first
#define se second
#define pb push_back
#define psf push_front
#define ppb pop_back
#define ppf pop_front
#define sz(x) (int)x.size()
#define mnto(x,y) x=min(x,(__typeof__(x))y)
#define mxto(x,y) x=max(x,(__typeof__(x))y)
#define INF 1023456789
#define LINF 1023456789123456789
#define all(x) x.begin(),x.end()
#define lb(x,v) (lower_bound(all(x),v)-x.begin())
#define ub(x,v) (upper_bound(all(x),v)-x.begin())
#define disc(x) sort(all(x));x.resize(unique(all(x))-x.begin());
typedef long long ll;
typedef long double ld;
typedef pair<int,int> ii;
typedef pair<ll,ll> pll;
typedef tuple<int,int,int> iii;
typedef tuple<int,int,int,int> iiii;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<ii> vii;
mt19937 rng(time(0));
#define mod 1000000007
inline int add(int a,int b){
int r=a+b;
while(r>=mod)r-=mod;
while(r<0)r+=mod;
return r;
}
inline int mult(int a,int b){
return (int)(((ll)(a*b))%mod);
}
inline int rd(){
int x=0;
char ch=getchar_unlocked();
while(!(ch&16))ch=getchar_unlocked();//keep reading while current character is whitespace
while(ch&16){//this will break when ‘\n’ or ‘ ‘ is encountered
x=(x<<3)+(x<<1)+(ch&15);
ch=getchar_unlocked();
}
return x;
}
#define maxn 500005
int n,a[maxn],l[maxn],r[maxn],s[maxn];
int main(){
sf("%d",&n);
ll ans=0;
for(int i=1;i<=n;++i){
sf("%d",&a[i]);
if(a[i]!=-1)ans+=a[i];
}
int p=-1,q=-1,y=-1;
for(int i=1;i<=n;++i){
if(a[i]==-1)p=i;
else if(p!=-1)l[i]=p;
}
p=-1;
for(int i=n;i>=1;--i){
if(a[i]==-1)p=i;
else if(a[i]){
if(p!=-1)r[i]=p;
if(q!=-1)s[i]=q;//next a[i]>0
else y=i;//last a[i]>0
q=i;
}
}
map<int,int> mp;
priority_queue<int> pq;
ll sum=0;
for(int i=1;i<=n;++i){
if(a[i]<=0)continue;
dbg("%d:\n",i);
int d=INF;
if(l[i])d=min(d,2*(i-l[i]));
if(r[i])d=min(d,2*(r[i]-i));
int f=d;//what is the cost if this guy does a walk?
if(s[i]&&r[i]){//have a prev, have a right
//if i have someone after me, walk is free but do one cycle
if(s[i]<r[i])f=min(f,2*(r[i]-s[i]));
//else if is last guy in range, one free walk
else f=0;
}
if(i==y)f=0;//if its the last guy, set to 0
--a[i];
while(a[i]&&sum+(i-1)<=2*(i-1)){//do cycles
mp[d]++;
dbg("take cycle %d\n",d);
sum+=d;
--a[i];
}
if(a[i]){//can replace a bigger cycle with this cycle
if(prev(mp.end())->fi>=d){
mp[d]+=a[i];
dbg("take a[i] more %d\n",d);
sum+=(ll)d*a[i];
}
while(!mp.empty()&&sum+(i-1)>2*(i-1)){
auto[a,b]=*prev(mp.end());
if(sum-(ll)a*b+(i-1)>2*(i-1)){
sum-=(ll)a*b;
mp.erase(prev(mp.end()));
dbg("remove %d %d\n",a,b);
}
else{
ll diff=sum+(i-1)-2*(i-1);
ll r=(diff-1)/a;
sum-=(ll)a*r;
mp[a]-=r;
dbg("remove2 %d %d\n",r,a);
break;
}
}
}
//try to add the cost of a walk?
if(sum+(i-1)>2*(i-1)){
if(prev(mp.end())->fi>=f){
auto[a,b]=*prev(mp.end());
sum-=a;
dbg("remove one %d\n",a);
if(--mp[a]==0)mp.erase(prev(mp.end()));
sum+=f;
mp[f]++;
dbg("take %d\n",f);
}
}
else{
sum+=f;
mp[f]++;
dbg("take2 %d\n",f);
}
}
for(auto[a,b]:mp)ans-=b;
cout<<ans;
}
