#include<iostream>
#include<string>
#include<iomanip>
#include<cmath>
#include<vector>
#include<algorithm>
#include<utility>

using namespace std;

#define int long long
#define endl "\n"

constexpr long long INF = (long long)1e18;
constexpr long long MOD = 1'000'000'007; 

struct fast_io {
	fast_io(){
		std::cin.tie(nullptr);
		std::ios::sync_with_stdio(false);
	};
} fio;

class union_find
{
	int  _setnum;
	vector<int> par, nume;
public:
	union_find(){
	}
	
	union_find(int x){
		par.resize(x);
		nume.resize(x);
		init();
	}
	
	~union_find(){
		//
		
	}
	
	void clear(){
		_setnum = 0;
		par.clear();
		nume.clear();
	}
	
	void init(){
		_setnum = par.size();
		for(int i = 0; i < par.size(); i++){
			par[i] = i;
			nume[i] = 1;
		}
	}
	
	void resize(int x){
		
		par.resize(x);
		nume.resize(x);
		init();
	}

	int find(int x){
		return par[x] == x ? x : par[x] = find(par[x]);
	}

	void unite(int x, int y){
		x = find(x);
		y = find(y);
	
		if(x == y)return;
		
		_setnum--;
		
		if(nume[x] > nume[y]) std::swap(x,y);
		
		par[x] = y;
		nume[y] += nume[x];
	}
	
	bool same(int x, int y){
		return find(x) == find(y);
	}
	
	int numel(int x){
		return nume[find(x)];
	}
	
	int size(){
		return par.size();
	}
	
	int setnum(){
		return _setnum;
	}
};

signed main(){
	cout<<fixed<<setprecision(10);
	
	vector<vector<int>> ind(100000);
	vector<int> A;
	int ans = 0;
	int N;
	union_find uf;
	
	cin>>N;
	
	A.resize(N);
	uf.resize(N);
	
	for(int i = 0; i < N; i++){
		cin>>A[i];
		
		A[i]--;
		
		ind[A[i]].push_back(i);
	}
	
	for(int i = 0; i < ind.size(); i++){
		
		if(ind[i].size() <= 1) continue;
		
		if(ind[i].front() == 0 && ind[i].back() == N-1) {
			uf.unite(0, N-1);
			
			ans = 1;
		}
		
		for(int j = 1; j < ind[i].size(); j++){
			if(ind[i][j-1] + 1 == ind[i][j]) {
				uf.unite(ind[i][j-1], ind[i][j]);
			}
		}
		
		if(uf.numel(ind[i][0]) != ind[i].size()) {
			ans = -1;
			break;
		}
	}
	
	cout<<ans<<endl;
	
	return 0;
}