#include <bits/stdc++.h>
// #include <bits/extc++.h>
// #pragma GCC optimize("Ofast,unroll-loops,O3")
// #pragma GCC target("avx,avx2,fma")
using namespace std;
// using namespace __gnu_pbds;
// using namespace __gnu_cxx;
// template <typename T>
// using ordered_set = tree <T, null_type, less <T>, rb_tree_tag, tree_order_statistics_node_update>;
// template <typename T>
// using ordered_multiset = tree <T, null_type, less_equal <T>, rb_tree_tag, tree_order_statistics_node_update>;
using ll = long long;
using ld = long double;
using ull = unsigned long long;
using uint = unsigned int;
using i128 = __int128_t;
istream& operator>>(istream& is, i128& x) {
long long a;
is >> a;
x = (i128) a;
return is;
}
ostream& operator<<(ostream& os, i128& x) {
long long a = (long long) x;
os << a;
return os;
}
template <typename T>
ostream& operator<<(ostream& is, vector <T>& a) {
for (uint i = 0; i < a.size(); ++i) is << a[i] << " ";
return is;
}
#ifdef LOCAL
# define DEBUG(x) cerr << "(" << __LINE__ << ") " << #x << ": " << x << endl;
#else
# define DEBUG(x)
#endif
const ll inf = 1e18 + 1e16;
const int inf_t = 1e9 + 7;
const ll mod = 1e18 + 7;
#define int long long
const int MAXN = 302;
int dp[2 * MAXN][MAXN][MAXN + 1];
signed main() {
#ifdef LOCAL
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
int n;
cin >> n;
vector <int> a(n);
for (int i = 0; i < n; ++i) cin >> a[i];
int nxt[n];
int prev[n];
nxt[n - 1] = inf_t;
for (int i = n - 2; i >= 0; --i) {
nxt[i] = nxt[i + 1];
if (a[i + 1] == -1) {
nxt[i] = i + 1;
}
}
prev[0] = -inf_t;
for (int i = 1; i < n; ++i) {
prev[i] = prev[i - 1];
if (a[i - 1] == -1) {
prev[i] = i - 1;
}
}
int s = 0;
for (int x : a) {
if (x != -1) s += x;
}
for (int &x : a) {
x = min(x, n);
}
for (int time = 0; time < 2 * n; ++time) {
for (int place = (time + 1) / 2; place < n; ++place) {
if (a[place] <= 0) continue;
for (int ost = a[place]; ost >= 0; --ost) {
dp[time][place][ost] = -inf_t;
}
}
}
int st = 0;
while (st < n && a[st] <= 0) ++st;
if (st < n) dp[st][st][a[st] - 1] = 1;
for (int time = st; time < 2 * n; ++time) {
for (int place = (time + 1) / 2; place < n; ++place) {
if (a[place] <= 0) continue;
for (int ost = a[place] - 1; ost >= 0; --ost) {
if (dp[time][place][ost] < 0) continue;
for (int to = place; to < n; ++to) {
if (a[to] <= 0) continue;
int diff = inf_t;
int i = nxt[place];
int j = prev[to];
if (i <= j) {
diff = to - place;
} else {
diff = min(i + i - place - to, place + to - j - j);
}
int cur = a[to] - 1;
if (to == place) {
if (ost == 0) continue;
cur = ost - 1;
}
if (diff + time < 2 * n) {
dp[diff + time][to][cur] = max(dp[diff + time][to][cur], dp[time][place][ost] + 1);
}
}
}
}
}
int ans = 0;
for (int time = 0; time < 2 * n; ++time) {
for (int place = (time + 1) / 2; place < n; ++place) {
if (a[place] <= 0) continue;
for (int ost = a[place]; ost >= 0; --ost) {
ans = max(ans, dp[time][place][ost]);
}
}
}
cout << s - ans;
}
