#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <queue>
#include <string>
#include <map>
#include <set>
#include <stack>
#include <tuple>
#include <deque>
#include <array>
#include <numeric>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <chrono>
#include <random>
#include <limits>
#include <iterator>
#include <functional>
#include <sstream>
#include <fstream>
#include <complex>
#include <cstring>
#include <unordered_map>
using namespace std;

using ll = long long;
using P = pair<int, int>;
constexpr int INF = 1001001001;
constexpr int mod = 1000000007;
// constexpr int mod = 998244353;

template<class T>
inline bool chmax(T& x, T y){
    if(x < y){
        x = y;
        return true;
    }
    return false;
}
template<class T>
inline bool chmin(T& x, T y){
    if(x > y){
        x = y;
        return true;
    }
    return false;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int N, A;
    cin >> N;
    vector<vector<int>> pos(N + 1);
    bool solo = true;
    for(int i = 0; i < N; ++i){
        cin >> A;
        if(pos[A].size())   solo = false;
        pos[A].emplace_back(i);
    }
    if(solo){
        cout << 0 << endl;
        return 0;
    }

    for(int k = 1; k <= N; ++k){
        if(pos[k].size() <= 1)  continue;
        bool link = true, multi = false;
        int n = pos[k].size();
        for(int i = 0; i + 1 < n; ++i){
            if(pos[k][i] + 1 != pos[k][i + 1]){
                if(link)    link = false;
                else    multi = true;
            }
        }
        if(multi){
            cout << -1 << endl;
            return 0;
        }
        if(!link){
            if(pos[k][0] != 0 || pos[k][n - 1] != N - 1){
                cout << -1 << endl;
                return 0;
            }
        }
    }

    cout << 1 << endl;

    return 0;
}