#include <stdio.h>
#include <string.h>
#define N 5000
#define INF 0x3f3f3f3f3f3f3f3fLL
long long min(long long a, long long b) { return a < b ? a : b; }
int max(int a, int b) { return a > b ? a : b; }
long long eval(long long a, long long b, long long c) { return b - a * c; }
int main() {
static int aa[N], ll[N], rr[N], tt[N];
static long long dp[N][N * 2 + 1], dq[N][N * 2 + 1], dr[N * 2 + 1], qu[N * 2 + 1];
int n, i, j, k, head, cnt;
long long ans;
scanf("%d", &n);
for (i = 0; i < n; i++)
scanf("%d", &aa[i]);
for (i = 0; i < n; i++) {
for (j = i - 1; j >= 0; j--)
if (aa[j] == -1)
break;
ll[i] = j;
for (j = i + 1; j < n; j++)
if (aa[j] == -1)
break;
rr[i] = j;
tt[i] = min(ll[i] == -1 ? INF : i - ll[i], rr[i] == n ? INF : rr[i] - i) * 2;
}
for (i = 0; i < n; i++) {
memset(dp[i], 0x3f, (n * 2 + 1) * sizeof *dp[i]);
memset(dq[i], 0x3f, (n * 2 + 1) * sizeof *dq[i]);
}
ans = 0;
for (i = 0; i < n; i++)
if (aa[i] > 0) {
for (k = 0; k <= n * 2; k++) {
if ((j = ll[i]) >= 0)
dp[i][k] = min(dp[i][k], dq[j][k] + i - j);
if ((j = rr[i]) < n)
dp[i][k] = min(dp[i][k], dq[j][k] + j - i);
}
dp[i][1] = min(dp[i][1], i);
head = cnt = 0;
for (k = 0; k <= n * 2; k++) {
if (dp[i][k] <= i * 2) {
while (cnt && eval(qu[head + cnt - 1], dp[i][qu[head + cnt - 1]], tt[i]) >= eval(k, dp[i][k], tt[i]))
cnt--;
qu[head + cnt++] = k;
}
while (cnt && k - qu[head] >= aa[i])
head++, cnt--;
dr[k] = cnt == 0 ? INF : eval(qu[head], dp[i][qu[head]], tt[i]) + (long long) tt[i] * k;
}
for (k = 0; k <= n * 2; k++)
dp[i][k] = dr[k];
for (k = 0; k <= n * 2; k++) {
long long t = dp[i][k];
if (t > i * 2)
continue;
ans = max(ans, k);
if (k < n * 2) {
if ((j = ll[i]) >= 0)
dq[j][k + 1] = min(dq[j][k + 1], t + i - j);
if ((j = rr[i]) < n)
dq[j][k + 1] = min(dq[j][k + 1], t + j - i);
}
}
} else
for (k = 0; k <= n * 2; k++) {
long long t = dq[i][k];
if (t == INF)
continue;
if ((j = rr[i]) < n)
dq[j][k] = min(dq[j][k], t + j - i);
}
ans = -ans;
for (i = 0; i < n; i++)
if (aa[i] > 0)
ans += aa[i];
printf("%lld\n", ans);
return 0;
}
