#include<iostream>
#include<vector>
using namespace std;
int N;
bool ex[1<<17];
main()
{
	cin>>N;
	vector<pair<int,int> >A;
	int pre;
	bool out=false;
	for(int i=0;i<N;i++)
	{
		int a;cin>>a;
		if(i==0||pre!=a)
		{
			A.push_back(make_pair(a,1));
			pre=a;
			if(ex[a])
			{
				out=true;
			}
			ex[a]=true;
		}
		else A.back().second++;
	}
	if(!out)cout<<0<<endl;
	else cout<<(A.size()==3?1:-1)<<endl;
}