#include <fstream>
using namespace std;

ifstream fin("inaltime.in");
ofstream fout("inaltime.out");
int a[105][105], n, k, viz[105], c[105], niv[105], t[105];

void citire()
{
    int f, g;
    fin >> n >> k;
    for (int i = 1; i < n; ++i)
    {
        fin >> f >> g;
        a[f][g] = a[g][f] = 1;
    }
}

void bf(int x)
{
    int p,u,k;
    p=u=1;
    c[p]=x;
    viz[x]=1;
    niv[x]=1;
    while(p<=u)
    {
        k=c[p];
        p++;
        for(int i=1; i<=n; ++i)
        {
            if(t[i]==k && viz[i]==0)
            {
                u++;
                c[u]=i;
                viz[i]=1;
                niv[i]=niv[k]+1;
                //t[i]=k;
            }
        }
    }
}

int main()
{
    fin>>n;int radacina,maxi=-1;
    for(int i=1; i<=n; ++i)
    {
         fin>>t[i];
         if(t[i]==0)
            radacina = i;
    }
     bf(radacina);
     for(int i=1; i<=n; ++i)
     {
         if(niv[i]>maxi)
            maxi=niv[i];
     }
fout<<maxi;
return 0;
}