#include <bits/stdc++.h>
#define int long long
using namespace std;
const int nmax = 6e5 + 1, inf = 1e9;
int v[nmax];
int st[nmax];
int dr[nmax];
int nxt_val[nmax];
int32_t main() {
int n;
cin >> n;
for (int i = 1; i <= n; i++) {
cin >> v[i];
}
int ult = 0;
for (int i = 1; i <= n; i++) {
if (v[i] == -1) {
ult = i;
}
st[i] = ult;
}
ult = 0;
int special = -1;
int ult_bun = 0;
for (int i = n; i >= 1; i--) {
if (v[i] == -1) {
ult = i;
}
dr[i] = ult;
if (v[i]) {
if (ult_bun) {
nxt_val[i] = ult_bun;
} else {
special = i;
}
ult_bun = i;
}
}
map<int, int> dists;
int ans = 0, sum = 0;
for (int i = 1; i <= n; i++) {
if (v[i] <= 0) {
continue;
}
ans += v[i];
int dist = inf;
if (st[i]) {
dist = min(dist, (i - st[i]) * 2);
}
if (dr[i]) {
dist = min(dist, (dr[i] - i) * 2);
}
int ult_dist = dist;
if (dr[i] && nxt_val[i]) {
if (nxt_val[i] > dr[i]) {
ult_dist = 0;
} else {
ult_dist = min(ult_dist, (dr[i] - nxt_val[i]) * 2);
}
}
if (special == i) {
ult_dist = 0;
}
v[i]--; // rezerval pentru ult_dist
// adaug normal
while (v[i] && sum <= i - 1) {
sum += dist;
v[i]--;
dists[dist]++;
}
if (v[i]) {
dists[dist] += v[i];
sum += v[i] * dist;
while (dists.size() && sum > i - 1) {
auto [x, y] = *dists.rbegin();
if (sum - x * y > i - 1) {
dists.erase(x);
sum -= x * y;
} else {
int dif = sum - (i - 1);
int scot = (dif - 1) / x;
sum -= scot * x;
dists[x] -= scot;
if (!dists[x]) {
dists.erase(x);
}
break;
}
}
}
if (sum > i - 1) {
auto [x, y] = *dists.rbegin();
if (x >= ult_dist) {
dists[x]--;
if (!dists[x]) {
dists.erase(x);
}
sum -= x;
}
}
if (sum <= i - 1) {
sum += ult_dist;
dists[ult_dist]++;
}
}
for (auto it : dists) {
ans -= it.second;
}
cout << ans;
return 0;
}
