#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <cassert>
#include <fstream>
#include <utility>
#include <functional>
#include <time.h>
#include <stack>
#include <array>
#include <list>
#define popcount __builtin_popcount
using namespace std;
typedef long long int ll;
typedef pair<int, int> P;

int main()
{
	int n;
    cin>>n;
    int a[100010];
    vector<int> v[100010];
    for(int i=0; i<n; i++){
        cin>>a[i];
        a[i]--;
        v[a[i]].push_back(i);
    }
    int s[100010]={}, t=0;
    for(int i=0; i<n; i++){
        if(v[i].empty()) continue;
        t++;
        int c=0;
        for(int j=0; j<(int)v[i].size()-1; j++){
            if(v[i][j+1]-v[i][j]>1){
                c++;
            }
        }
        if(c>1 || (c==1 && (v[i][0]!=0 || v[i].back()!=n-1))){
            cout<<-1<<endl;
            return 0;
        }
        if(c==1){
            for(int j=0; j<(int)v[i].size()-1; j++){
                if(v[i][j+1]-v[i][j]>1){
                    s[v[i][j]+1]++;
                    s[v[i][j+1]+1]--;
                    break;
                }
            }
        }else{
            s[0]++;
            s[v[i][0]+1]--;
            s[v[i].back()+1]++;
        }
    }
    for(int i=1; i<=n; i++) s[i]+=s[i-1];
    if(s[0]==t){
        cout<<0<<endl;
        return 0;
    }
    for(int i=1; i<=n; i++){
        if(s[i]==t){
            cout<<1<<endl;
            return 0;
        }
    }
    cout<<-1<<endl;
	return 0;
}