#include<bits/stdc++.h>
using namespace std;
#define ALL(x) x.begin(),x.end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define INF 1000000000
#define mod 1000000007
using ll=long long;
const ll LINF=1001002003004005006ll;
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
// ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<class T>bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;}

signed main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    int n;cin>>n;
    vector<int> a(n);
    rep(i,n) cin>>a[i];

    set<int> st;
    rep(i,n){
        st.insert(a[i]);
    }
    if(st.size()==1){
        cout<<0<<endl;
        return 0;
    }

    bool f=(a[0]==a[n-1]);

    int l=0;
    while(a[0]==a[l]) l++;

    int r=n;
    if(f){
        while(a[r-1]==a[n-1]) r--;
    }

    vector<int> last(n+1,-1);
    last[a[0]]=l-1;
    for(int i=l;i<r;i++){
        if(last[a[i]]>=0 and last[a[i]]+1!=i){
            cout<<-1<<endl;
            return 0;
        }
        last[a[i]]=i;
    }
    cout<<f<<endl;
    return 0;
}