#include <bits/stdc++.h>
using namespace std;
typedef int ll;
typedef vector<ll> v;
typedef vector<v> vv;
typedef vector<vv> vvv;
typedef vector<vvv> vvvv;
typedef pair<ll, ll> p;
typedef vector<p> vp;
typedef vector<vp> vvp;
typedef vector<vvp> vvvp;
typedef pair<ll, p> tri;
typedef vector<tri> vtri;
typedef vector<vtri> vvtri;
typedef vector<vvtri> vvvtri;
typedef vector<bool> vb;
typedef vector<vb> vvb;
typedef vector<vvb> vvvb;
#define f first
#define s second
#define pb push_back
#define eb emplace_back
#define all(v) (v).begin(),(v).end()
#define setmax(a, b) a = (a > b? a : b)
const ll INF = 1e18;
const ll mod = 1e9 + 7;
ll solve(ll N, v &a, ll firstPos, ll secondPos, ll lastEaten, ll state, map<pair<p,p>,ll> &dp, v &firstParkLeft, v &firstParkRight){
if (firstPos >= 2 * N || lastEaten >= N) return 0;
if (dp.count({{firstPos, secondPos},{lastEaten, state}}) > 0) return dp[{{firstPos, secondPos},{lastEaten, state}}];
if (state == 0)
{
ll res = solve(N, a, firstPos, secondPos, lastEaten + 1, state, dp, firstParkLeft, firstParkRight);
if (a[lastEaten] == 1)
{
ll distance = abs(secondPos - lastEaten);
if ((firstPos + distance) / 2 <= lastEaten)
{
setmax(res, solve(N, a, firstPos + distance, lastEaten, lastEaten + 1, 1, dp, firstParkLeft, firstParkRight) + 1);
}
}
dp[{{firstPos, secondPos},{lastEaten, state}}] = res;
return res;
}
ll res = 0;
if (firstParkLeft[secondPos] > -1)
{
ll distance = abs(secondPos - firstParkLeft[secondPos]);
if (firstPos + distance < 2 * N)
setmax(res, solve(N, a, firstPos + distance, firstParkLeft[secondPos], lastEaten,0, dp, firstParkLeft, firstParkRight));
}
if (firstParkRight[secondPos] < N)
{
ll distance = abs(secondPos - firstParkRight[secondPos]);
if (firstPos + distance < 2 * N)
setmax(res, solve(N, a, firstPos + distance, firstParkRight[secondPos], lastEaten,0, dp, firstParkLeft, firstParkRight));
}
dp[{{firstPos, secondPos},{lastEaten, state}}] = res;
return res;
}
int main()
{
ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
ll N;
cin >> N;
v a(N);
for (ll i = 0; i < N; i++) cin >> a[i];
v firstParkLeft(N, -1), firstParkRight(N, N);
ll lastPark = -1;
for (ll i = 0; i < N; i++)
{
if (a[i] == -1) lastPark = i;
firstParkLeft[i] = lastPark;
}
lastPark = N;
for (ll i = N - 1; i >= 0; i--)
{
if (a[i] == -1) lastPark = i;
firstParkRight[i] = lastPark;
}
map<pair<p, p>, ll> dp;
ll res = solve(N, a, 0, 0, 0, 0, dp, firstParkLeft, firstParkRight);
ll sum = 0;
for (auto x : a) if (x > 0) sum += x;
cout << sum - res << "\n";
return 0;
}
