#include <iostream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstdlib>
#include <cassert>
#include <vector>
#include <list>
#include <stack>
#include <queue>
#include <deque>
#include <map>
#include <set>
#include <bitset>
#include <string>
#include <algorithm>
#include <utility>
#define llint long long
#define inf 1e18
#define rep(x, s, t) for(llint (x) = (s); (x) < (t); (x)++)
#define Rep(x, s, t) for(llint (x) = (s); (x) <= (t); (x)++)
#define chmin(x, y) (x) = min((x), (y))
#define chmax(x, y) (x) = max((x), (y))

using namespace std;
typedef pair<llint, llint> P;

llint n;
llint a[200005];
map<llint, llint> mp;

int main(void)
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	cin >> n;
	for(int i = 1; i <= n; i++) cin >> a[i];
	
	for(int i = 2; i <= n; i++){
		if(a[i] != a[i-1]) mp[a[i-1]]++;
	}
	mp[a[n]]++;
	
	for(auto it = mp.begin(); it != mp.end(); it++){
		if(it->second <= 1) continue;
		if(it->second >= 3){
			cout << -1 << endl;
			return 0;
		}
		if(a[1] == it->first && a[n] == it->first) continue;
		cout << -1 << endl;
		return 0;
	}
	
	if(a[1] == a[n] && mp.size() >= 2) cout << 1 << endl;
	else cout << 0 << endl;
	
	return 0;
}