#include<iostream>

#include<cstdio>

#include<cstdlib>

#include<cmath>

#include<ctime>

#include<cctype>

#include<string>

#include<cstring>

#include<algorithm>

using namespace std;

const int N=1e5+;

int n,num[N],Left[N],Right[N],deep,stack[N],ans;

int main()

{

  //freopen("art2.in","r",stdin);

 // freopen("art2.out","w",stdout);

  scanf("%d",&n);

  for(int i=;i<=n;i++)

    Left[i]=1e+;

  for(int i=;i<=n;i++)

  {

    scanf("%d",&num[i]);

    Left[num[i]]=min(Left[num[i]],i);

    Right[num[i]]=max(Right[num[i]],i);

  }

  for(int i=;i<=n;i++)

  {

    if(!num[i])  continue;

    if(Left[num[i]]==i)

    {

      stack[++deep]=num[i];

      ans=max(deep,ans);

    }

    if(Right[num[i]]==i)

    {

        if(stack[deep]!=num[i])

        {

          cout<<"-1"<<endl;

          return ;

        }

        else

          deep--;

    }

  }

  cout<<ans<<endl;

  return ;

}