#include <cstdio>
#include <vector>
#include <cstring>
#include <algorithm>
using namespace std;
int n, a[500006], dist[500006], res, dp[306][606][306][2];
vector<int> playgrounds;
int f(int x, int y, int z, int w) {
if (dp[x][y][z][w] + 1) return dp[x][y][z][w];
dp[x][y][z][w] = 0;
if (2 * x >= y && x >= z && !w && a[x] > 0) dp[x][y][z][w] = max(dp[x][y][z][w], f(x, y, x + 1, 1));
if (w && a[x] == -1) dp[x][y][z][w] = max(dp[x][y][z][w], f(x, y, z, 0) + 1);
if (y < 2 * n - 1) {
if (x > 0) dp[x][y][z][w] = max(dp[x][y][z][w], f(x - 1, y + 1, z, w));
if (x < n - 1) dp[x][y][z][w] = max(dp[x][y][z][w], f(x + 1, y + 1, z, w));
} else dp[x][y][z][w] = max(dp[x][y][z][w], w);
return dp[x][y][z][w];
}
int main() {
memset(dp, -1, sizeof dp);
scanf("%d", &n);
long long sum = 0;
for (int i = 0; i < n; i++) {
scanf("%d", a + i);
sum += max(a[i], 0);
dist[i] = (int)1e9;
if (a[i] < 0) playgrounds.push_back(i);
}
{
int tmp = (int)1e9;
for (int i = 0; i < n; i++) {
if (a[i] == -1) tmp = 0;
dist[i] = min(dist[i], tmp);
tmp++;
}
tmp = (int)1e9;
for (int i = n - 1; i >= 0; i--) {
if (a[i] == -1) tmp = 0;
dist[i] = min(dist[i], tmp);
tmp++;
}
}
printf("%lld", sum - f(0, 0, 0, 0));
}
