#include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> pii;
const int maxn = 500010, offset = 1 << 19, INF = 1e9;
int n, a[maxn], l[maxn], r[maxn], before[maxn];
unordered_map<int, int> specialChair;
struct SEG {
vector<int> t, p;
SEG() { t.resize(2 * offset), p.resize(2 * offset); }
void init() {
for (int i = offset; i < 2 * offset; i++) {
t[i] = INF + (i - offset);
}
for (int i = offset - 1; i; i--) {
t[i] = min(t[i * 2], t[i * 2 + 1]);
}
}
void prop(int x) {
if (p[x]) {
t[x] += p[x];
if (x < offset) {
p[x * 2] += p[x];
p[x * 2 + 1] += p[x];
}
p[x] = 0;
}
}
void upd(int a, int b, int v, int x = 1, int lo = 0, int hi = offset) {
if (lo >= a && hi <= b) { p[x] += v, prop(x); return; }
prop(x);
if (lo >= b || hi <= a) return;
int mi = (lo + hi) >> 1;
upd(a, b, v, x * 2, lo, mi);
upd(a, b, v, x * 2 + 1, mi, hi);
t[x] = min(t[x * 2], t[x * 2 + 1]);
}
int get(int a, int b, int x = 1, int lo = 0, int hi = offset) {
if (lo >= b || hi <= a) return INF;
prop(x);
if (lo >= a && hi <= b) return t[x];
int mi = (lo + hi) >> 1;
return min(get(a, b, x * 2, lo, mi), get(a, b, x * 2 + 1, mi, hi));
}
} special, updates, tracker;
void moveSpecial(int a, int b) {
special.upd(a, a + 1, +INF);
special.upd(b, b + 1, -INF);
}
void addSpecial(int a) { special.upd(a, a + 1, -INF); }
long long sum;
void solve(int x, int d, int type) {
int cnt = 0;
while (a[x] > 0) {
if (tracker.get(x, offset) < 2 * d) break;
if (x - updates.get(x, x + 1) < type * d * 2) break;
if (r[x] != -1) {
int spec = specialChair[r[x]];
if (type) spec = min(spec, x);
if (a[spec] == 1 && spec == x) {
if (!type) break;
spec = before[spec];
}
if (spec <= l[x]) break;
if (a[spec] <= 0) break;
moveSpecial(specialChair[r[x]], spec);
special.upd(x + type, offset, -2 * d);
int diff = special.get(0, offset);
if (diff < 0) {
special.upd(x + type, offset, 2 * d);
moveSpecial(spec, specialChair[r[x]]);
break;
}
specialChair[r[x]] = spec;
}
if (cnt == 0) tracker.upd(x, x + 1, -INF - 2 * d * (type - 1));
tracker.upd(x, offset, -2 * d);
updates.upd(x, offset, 2 * d);
a[x]--, sum--, cnt++;
}
}
int main() {
ios::sync_with_stdio(0), cin.tie(0);
int n;
cin >> n;
for (int i = 0; i < n; i++) {
cin >> a[i];
if (a[i] != -1) sum += a[i];
}
int tmp = -1;
for (int i = 0; i < n; i++) {
if (a[i] == -1) tmp = i;
l[i] = tmp;
}
tmp = -1;
for (int i = n - 1; i >= 0; --i) {
if (a[i] == -1) tmp = i;
r[i] = tmp;
}
tmp = -1;
for (int i = 0; i < n; i++) {
before[i] = tmp;
if (a[i] > 0) tmp = i;
}
special.init(), tracker.init();
for (int i = 0; i < n; i++) {
if (a[i] > 0 && r[i] != -1 && before[r[i]] == i) {
addSpecial(i);
specialChair[r[i]] = i;
}
}
vector<pair<int, pii>> cand;
for (int i = 0; i < n; i++) if (a[i] > 0) {
if (r[i] != -1) cand.push_back({r[i] - i, {i, 1}});
if (l[i] != -1) cand.push_back({i - l[i], {i, 0}});
}
sort(cand.begin(), cand.end());
for (auto &p : cand) {
solve(p.second.first, p.first, p.second.second);
}
for (int i = 0; i < n; i++) {
if (a[i] > 0 && r[i] != -1) a[r[i]] = -2;
if (a[i] == -2) sum--;
}
cout << sum << "\n";
return 0;
}
