#include <bits/stdc++.h>
using namespace std;
const int nx=305;
int n, a[nx], l[nx], r[nx], lst=-1, dp[nx][nx], res, cnt;
int cost(int a, int b)
{
int mn=1e9;
if (l[a]!=-1) mn=min(mn, abs(a-l[a])+abs(b-l[a]));
if (r[a]!=-1) mn=min(mn, abs(a-r[a])+abs(b-r[a]));
return mn;
}
int main()
{
cin.tie(NULL)->sync_with_stdio(false);
cin>>n;
for (int i=1; i<=n; i++) cin>>a[i];
for (int i=1; i<=n; i++)
{
if (a[i]==-1) lst=i;
if (a[i]>0) l[i]=lst, cnt+=a[i];
}
lst=-1;
for (int i=n; i>=1; i--)
{
if (a[i]==-1) lst=i;
if (a[i]>0) r[i]=lst;
}
for (int i=1; i<=n; i++)
{
if (a[i]>0)
{
for (int j=1; j<=n; j++) dp[i][j]=1e9;
dp[i][1]=i-1;
for (int j=1; j<i; j++) if (a[j]>0) for (int k=1; k<=i; k++) if (dp[j][k]+cost(j, i)<=2*(i-1)) dp[i][k+1]=min(dp[i][k+1], dp[j][k]+cost(j, i));
for (int j=2; j<=min(n-1, a[i]); j++)
{
for (int k=n-1; k>=1; k--) if (dp[i][k]+cost(i, i)<=2*(i-1)) dp[i][k+1]=min(dp[i][k+1], dp[i][k]+cost(i, i));
}
for (int j=1; j<=i; j++)
{
if (dp[i][j]<1e9) res=max(res, j);
}
}
}
cout<<cnt-res;
}
