/**
 *    author:  otera    
**/
#include<iostream>
#include<string> 
#include<cstdio>
#include<cstring>
#include<vector>
#include<cmath>
#include<algorithm> 
#include<functional>
#include<iomanip>
#include<queue>
#include<deque>
#include<ciso646>
#include<random>
#include<map>
#include<set>
#include<complex>
#include<bitset>
#include<stack>
#include<unordered_map>
#include<utility>
#include<cassert>
using namespace std;

//#define int long long
typedef long long ll;
typedef unsigned long long ul;
typedef unsigned int ui;
typedef long double ld;
const int inf=1e9+7;
const ll INF=1LL<<60 ;
const ll mod=1e9+7 ;
#define rep(i,n) for(int i=0;i<n;i++)
#define per(i,n) for(int i=n-1;i>=0;i--)
#define Rep(i,sta,n) for(int i=sta;i<n;i++)
#define rep1(i,n) for(int i=1;i<=n;i++)
#define per1(i,n) for(int i=n;i>=1;i--)
#define Rep1(i,sta,n) for(int i=sta;i<=n;i++)
typedef complex<ld> Point;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<int, int> P;
typedef pair<ld, ld> LDP;
typedef pair<ll, ll> LP;
#define fr first
#define sc second
#define all(c) c.begin(),c.end()
#define pb push_back
#define debug(x)  cerr << #x << " = " << (x) << endl;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

void solve() {
	int n; cin >> n;
    vector<int> a(n);
    set<int> se;
    rep(i, n) {
        cin >> a[i];
    }
    bool ok = 1;
    se.insert(a[0]);
    for(int i = 0; i < n - 1; ++ i) {
        if(a[i] != a[i + 1]) {
            //
            if(se.find(a[i + 1]) != se.end()) {
                ok = 0;
            }
            //
            se.insert(a[i + 1]);
            
        }
    }
    if(ok) {
        cout << 0 << endl;
        return;
    }
    if(a[0] == a[n - 1]) {
        int left = 0, right = n - 1;
        while(left + 1 < n && a[left + 1] == a[0]) ++ left;
        while(right - 1 >= 0 && a[right - 1] == a[n - 1]) -- right;
            set<int> se1;
            for(int j = left + 1; j < right - 1; ++ j) {
                if(a[j] == a[0]) {
                    cout << -1 << endl;
                    return;
                }
                if(a[j] != a[j + 1]) {
                    //
                    if(se1.find(a[j + 1]) != se1.end()) {
                        cout << -1 << endl;
                        return;
                    }
                    //
                    se1.insert(a[j + 1]);
                    if(j == left + 1) se1.insert(a[j]);
                }
            }
            cout << 1 << endl;
        }
    else {
        cout << -1 << endl;
    }
}

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	//cout << fixed << setprecision(10);
	//int t; cin >> t; rep(i, t)solve();
	solve();
    return 0;
}