#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
constexpr int INF = 1e9 + 100;
int a[500500];
struct Seg {
int tree[2002000], lazy[2002000];
void init(int i, int l, int r, const vector<int> &V) {
lazy[i] = 0;
if (l==r) {tree[i] = V[l]; return;}
int m = (l+r)/2;
init(i<<1, l, m, V);
init(i<<1|1, m+1, r, V);
tree[i] = min(tree[i<<1], tree[i<<1|1]);
}
void propagate(int i, int l, int r) {
tree[i] += lazy[i];
if (l!=r) lazy[i<<1] += lazy[i], lazy[i<<1|1] += lazy[i];
lazy[i] = 0;
}
void update(int i, int l, int r, int s, int e, int x) {
propagate(i, l, r);
if (r<s || e<l) return;
if (s<=l && r<=e) {
lazy[i] += x;
propagate(i, l, r);
return;
}
int m = (l+r)/2;
update(i<<1, l, m, s, e, x);
update(i<<1|1, m+1, r, s, e, x);
tree[i] = min(tree[i<<1], tree[i<<1|1]);
}
void set(int i, int l, int r, int p, int x){
propagate(i, l, r);
if (r<p || p<l) return;
if (l==r) {
tree[i] = x;
return;
}
int m = (l+r)/2;
set(i<<1, l, m, p, x);
set(i<<1|1, m+1, r, p, x);
tree[i] = min(tree[i<<1], tree[i<<1|1]);
}
int query(int i, int l, int r, int s, int e) {
propagate(i, l, r);
if (r<s || e<l) return INF;
if (s<=l && r<=e) return tree[i];
int m = (l+r)/2;
return min(query(i<<1, l, m, s, e), query(i<<1|1, m+1, r, s, e));
}
}tree, tree2;
int main() {
int n;
scanf("%d", &n);
for (int i=1;i<=n;i++) scanf("%d", a+i);
a[0] = a[n+1] = -1;
ll ans = 0;
for (int i=1;i<=n;i++) ans += max(a[i], 0);
if (ans==0) {
printf("0\n");
return 0;
}
priority_queue<array<int, 5>, vector<array<int, 5>>, greater<array<int, 5>>> pq;
vector<int> V, W(n+1, INF), chk(n+1);
vector<vector<int>> L;
for (int i=0,j=0;i<=n;i=j) {
if (a[i]!=-1){j = i+1; continue;}
j = i+1;
while(j <= n && a[j]!=-1) j++;
int ti = i==0?-INF:i, tj = j==n+1?INF:j;
int r = -1, mx = -1, idx = -1;
for (int k=i+1;k<j;k++) if (a[k] > 0) {
r = k;
if (mx < min(k-ti, tj-k)) {
mx = min(k-ti, tj-k);
idx = k;
}
}
if (r < 0) continue;
V.push_back(-1);
L.emplace_back();
W[r] = r-1;
chk[r] = 1;
ans--;
for (int k=i+1;k<j;k++) if (a[k] > 0){
L.back().push_back(k);
int c = k==idx?a[k]-1:a[k];
if (c==0) continue;
if (k-ti <= tj-k){
pq.push({k-ti, c, k, (int)V.size()-1, 0});
}
else{
pq.push({tj-k, c, k, (int)V.size()-1, 1});
}
}
}
int sz = V.size();
tree.init(1, 0, n, W);
tree2.init(1, 0, sz-1, V);
while(!pq.empty()) {
auto [w, c, x, i, typ] = pq.top(); pq.pop();
if (typ==0 && !chk[x]){
int v = tree2.query(1, 0, sz-1, i, i) + x;
if (v < 0) continue;
if (tree.query(1, 0, n, x+1, n) < w*2) continue;
chk[x] = 1;
tree.set(1, 0, n, x, v);
tree.update(1, 0, n, x+1, n, -w*2);
tree2.update(1, 0, sz-1, i, sz-1, -w*2);
if (c > 1) pq.push({w, c-1, x, i, typ});
}
else if (typ==1 && c==1 && L[i].size() >= 2){
int y = L[i].end()[-2];
int v = tree2.query(1, 0, sz-1, i, i) + y;
if (v < 0) continue;
if (tree.query(1, 0, n, y+1, n) < w*2) continue;
chk[y] = 1;
tree.set(1, 0, n, y, v);
tree.update(1, 0, n, y+1, n, -w*2);
tree2.update(1, 0, sz-1, i+1, sz-1, -w*2);
L[i].pop_back();
}
else{
if (tree.query(1, 0, n, x, n) < w*2) continue;
tree.update(1, 0, n, x, n, -w*2);
tree2.update(1, 0, sz-1, i+typ, sz-1, -w*2);
if (c > 1) pq.push({w, c-1, x, i, typ});
}
ans--;
}
printf("%lld\n", ans);
}
