#include<bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define rrep(i,n) for (int i = (n)-1; i >= 0; i--)
#define rep2(i,s,n) for (int i = (s); i < (n); ++i)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define pb push_back
#define eb emplace_back
#define vi vector<int>
#define vvi vector<vector<int>>
#define vl vector<ll>
#define vvl vector<vector<ll>>
#define vd vector<double>
#define vs vector<string>
#define vc vector<char>
#define vb vector<bool>
#define vp vector<P>
using namespace std;
using ll = long long;
using P = pair<int,int>;
using LP = pair<ll,ll>;
template<class T> istream& operator>>(istream& is,vector<T>& v) { for(T& t:v){is>>t;} return is; }
template<class T> ostream& operator<<(ostream& os,const vector<T>& v) { for(T t:v){os<<t<<" ";} os<<"\n"; return os; }
void Yes(bool b) { cout << (b ? "Yes" : "No") << endl; }
void YES(bool b) { cout << (b ? "YES" : "NO") << endl; }
template<class T> inline bool chmin(T& a,T b) {if(a > b){a = b; return true;} return false;}
template<class T> inline bool chmax(T& a,T b) {if(a < b){a = b; return true;} return false;}
const int inf = 1001001001;
const ll linf = 1001001001001001001;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr); cout.tie(nullptr);
    int n;
    cin >> n;
    vi a(n);
    cin >> a;
    rep(i,n) a[i]--;
    vi last(n,-2);
    bool flag = false;
    rep(i,n) {
        if(last[a[i]] == -2 || last[a[i]] == i-1) {
            last[a[i]] = i;
        } else {
            if(a[i] == a[0]) {
                if(!flag) {
                    flag = true;
                    last[a[i]] = i;
                } else {
                    cout << -1 << endl;
                    return 0;
                }
            } else {
                cout << -1 << endl;
                return 0;
            }
        }
    }
    if(flag) {
        if(a[0] == a[n-1]) cout << 1 << endl;
        else cout << -1 << endl;
    } else {
        cout << 0 << endl;
    }
}