//#define _GLIBCXX_DEBUG
#include<bits/stdc++.h>
#define PI 3.14159265359
using namespace std;
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
#define bit(n,k) (((ll)n>>(ll)k)&1) /*nのk bit目*/
const long long INF= 1e+18+1;
typedef long long ll;
typedef vector<ll> vl;
typedef vector<vector<ll> >vvl;
typedef pair<ll,ll> P;
typedef tuple<ll,ll,ll> T;
//const ll MOD=1000000007LL;
const ll MOD=998244353LL;
string abc="abcdefghijklmnopqrstuvwxyz";
string ABC="ABCDEFGHIJKLMNOPQRSTUVWXYZ";
vl dx={-1,-1,-1,0,0,1,1,1};
vl dy={1,-1,0,1,-1,1,0,-1};

int main(){
  ll n;cin>>n;
  vl flag(114514);
  vl a(n);
  rep(i,n)cin>>a[i];
  bool ok=true;
  rep(i,n){
    if(i!=n-1){
      if(a[i+1]==a[i])continue;
    }
    if(flag[a[i]])ok=false;
    flag[a[i]]=1;
  }
  if(ok){
    cout<<0<<endl;
    return 0;
  }
  deque<ll>q;
  rep(i,n){
    q.push_back(a[i]);
  }
  while(q.front()==q.back()){
    ll d=q.front();
    q.pop_front();
    q.push_back(d);
  }
  ok=true;
  rep(i,114514)flag[i]=0;
  rep(i,n){
    if(i!=n-1){
      if(q[i+1]==q[i])continue;
    }
    if(flag[q[i]])ok=false;
    flag[q[i]]=1;
  }
  if(ok){
    cout<<1<<endl;
    return 0;
  }
  cout<<-1<<endl;
}