#include <bits/stdc++.h>
using namespace std;
#define F first
#define S second
#define R cin>>
#define Z class
#define ll long long
#define ln cout<<'\n'
#define in(a) insert(a)
#define pb(a) push_back(a)
#define pd(a) printf("%.10f\n",a)
#define mem(a) memset(a,0,sizeof(a))
#define all(c) (c).begin(),(c).end()
#define iter(c) __typeof((c).begin())
#define rrep(i,n) for(ll i=(ll)(n)-1;i>=0;i--)
#define REP(i,m,n) for(ll i=(ll)(m);i<(ll)(n);i++)
#define rep(i,n) REP(i,0,n)
#define tr(it,c) for(iter(c) it=(c).begin();it!=(c).end();it++)
ll check(ll n,ll m,ll x,ll y){return x>=0&&x<n&&y>=0&&y<m;}void pr(){ln;}
template<Z A,Z...B>void pr(const A &a,const B&...b){cout<<a<<(sizeof...(b)?" ":"");pr(b...);}
template<Z A>void PR(A a,ll n){rep(i,n){if(i)cout<<' ';cout<<a[i];}ln;}
const ll MAX=1e9+7,MAXL=1LL<<61,dx[8]={-1,0,1,0,-1,-1,1,1},dy[8]={0,1,0,-1,-1,1,1,-1};
typedef pair<ll,ll> P;

void Main() {
  ll n;
  R n;
  ll a[n];
  rep(i,n) R a[i];
  vector<ll> v;
  ll x=0;
  rep(i,n) {
    if(a[i]!=x) v.pb(a[i]);
    x=a[i];
  }
  map<ll,ll> m;
  ll c=0;
  rep(i,v.size()) m[v[i]]++;
  tr(it,m) {
    if(it->S>=2) {
      c++;
      x=it->F;
    }
  }
  if(!c) pr(0);
  else if(c==1) {
    ll ans=2;
    if(v[0]==x) ans--;
    if(v.back()==x) ans--;
    pr(ans);
  } else pr(-1);
}

int main(){ios::sync_with_stdio(0);cin.tie(0);Main();return 0;}