#include<fstream>
#include<set>

using namespace std;

const short nmax=5005;
int a[nmax];
short b[nmax],l[nmax],n,k;
set<short> s[nmax],generala,aux;
set<short>::iterator it;

int main(void){
    ifstream fin("secv.in");
    ofstream fout("secv.out");
    short i,j,max,poz,q;
    fin>>n; 
    for(i=1;i<=n;++i){
              fin>>a[i]; generala.insert(a[i]); }
    k=generala.size();                  
    b[n]=1; l[n]=0;
    for(i=1;i<=n;++i)s[i].insert(a[i]); 
    for(i=n-1;i>=1;--i){
                     max=0; aux.clear(); q=0;
                     for(j=i+1;j<=n;++j)
                       if(a[i]<a[j] && max<b[j] && s[i].count(a[j])==0)   
                            { max=b[j]; aux.insert(a[j]); q=j;}
                     b[i]=max+1; l[i]=q;
                     for(it=aux.begin();it!=aux.end();++it)
                           s[i].insert(*it);
                     }  
    poz=0;              
    for(i=1;i<=n;++i)if(b[i]==k) { poz=i; break; }
    if(poz==0)fout<<"-1";
      else{
           j=poz; max=0;
           while(l[j]>0) { j=l[j]; if(j>max)max=j; }
           fout<<max-poz+1;
           } 
    fout.close();                     
 return 0;            
  
}