#include <bits/stdc++.h>
#define endl '\n'
using namespace std;
typedef long long ll;
const ll MAXN = 1LL * (5e6 + 10LL);
const ll INF = 1LL * (4e18 + 10LL);
ll n;
ll a[MAXN];
ll lpg[MAXN];
ll rpg[MAXN];
ll after[MAXN];
ll sum;
ll breakpoint;
void read()
{
cin >> n;
for(int i = 1LL; i <= n; i++)
{
cin >> a[i];
if(a[i] != -1LL)
sum += a[i];
}
}
void fill_pgs()
{
breakpoint = -1LL;
ll last = -1LL;
for(ll i = 1LL; i <= n; i++)
{
if(a[i] == -1LL)
last = i;
else if(last != -1LL)
lpg[i] = last;
}
last = -1LL;
ll last2 = -1LL;
for(ll i = n; i >= 1LL; i--)
{
if(a[i] == -1LL)
last = i;
else if(a[i])
{
if(last != -1)
rpg[i] = last;
if(last2 != -1)
after[i] = last2;
else
breakpoint = i;
last2 = i;
}
}
}
void solve()
{
fill_pgs();
ll ans = 0LL;
map < ll, ll > mp;
for(ll i = 1; i <= n; i++)
{
if(a[i] <= 0)
continue;
ll tim = INF;
if(lpg[i])
tim = min(tim, 1LL * 2LL * (i - lpg[i]));
if(rpg[i])
tim = min(tim, 1LL * 2LL * (rpg[i] - i));
ll tmp = tim;
if(after[i] && rpg[i])
{
tmp = (after[i] < rpg[i] ? min(tmp, 1LL * 2LL * (rpg[i] - after[i])) : 0LL);
}
if(i == breakpoint)
tmp = 0LL;
a[i]--;
while(a[i] && ans <= i - 1)
{
mp[tim]++;
ans += tim;
a[i]--;
}
if(a[i])
{
if(prev(mp.end()) -> first >= tim)
{
mp[tim] += a[i];
ans += (tim * a[i]);
}
while(!mp.empty() && ans > i - 1)
{
auto [x, y] = *prev(mp.end());
if(ans - 1 * x * y > i - 1)
{
ans -= 1LL * (x * y);
mp.erase(prev(mp.end()));
}
else
{
ll diff = ans - (i - 1LL);
ll z = 1LL * (diff - 1LL) / x;
ans -= 1LL * (x * z);
mp[x] -= z;
break;
}
}
}
if(ans > i - 1)
{
if(prev(mp.end()) -> first >= tmp)
{
auto [x, y] = *prev(mp.end());
ans -= x;
mp[x]--;
if(!mp[x])
mp.erase(prev(mp.end()));
ans += tmp;
mp[tmp]++;
}
}
else
{
ans += tmp;
mp[tmp]++;
}
}
for(auto it = mp.begin(); it != mp.end(); it++)
{
sum -= (it -> second);
}
cout << sum << endl;
}
int main()
{
ios_base :: sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
read();
solve();
return 0;
}
