#include <iostream>
#include<vector>
#include<algorithm>
#include<string>
#include<map>
#include<set>
#include<stack>
#include<queue>
#include<math.h>
using namespace std;
typedef long long ll;
#define int long long
#define double long double
typedef vector<int> VI;
typedef pair<int, int> pii;
typedef vector<pii> VP;
typedef vector<string> VS;
typedef priority_queue<int> PQ;
template<class T>bool chmax(T &a, const T &b) { if (a < b) { a = b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b < a) { a = b; return 1; } return 0; }
#define fore(i,a) for(auto &i:a)
#define REP(i,n) for(int i=0;i<n;i++)
#define eREP(i,n) for(int i=0;i<=n;i++)
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define eFOR(i,a,b) for(int i=(a);i<=(b);++i)
#define SORT(c) sort((c).begin(),(c).end())
#define rSORT(c) sort((c).rbegin(),(c).rend())
#define LB(x,a) lower_bound((x).begin(),(x).end(),(a))
#define UB(x,a) upper_bound((x).begin(),(x).end(),(a))
#define INF 1000000000
#define LLINF 9223372036854775807
#define mod 1000000007
#define eps 1e-12 
//priority_queue<int,vector<int>, greater<int> > q2;



signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	int N; cin >> N;
	VI A(N);
	REP(i, N)cin >> A[i];
	VI B;
	REP(i, N) if (i == N - 1 || A[i] != A[i + 1])B.push_back(A[i]);

	VI cnt(N + 1, 0);
	REP(i, B.size()) {
		if (++cnt[B[i]] == 3) {
			cout << -1 << endl;
			return 0;
		}
	}

	int L = 0, R = B.size() - 1;
	int ans = 0;
	while (L < R) {
		if (cnt[B[L]] == 2 && cnt[B[R]] == 2) {
			if (B[L] == B[R]) {
				L++, R--;
				ans++;
			}
			else {
				cout << -1 << endl;
			}
		}
		else if (cnt[B[L]] == 2) {
			R--;
		}
		else if(cnt[B[R]] == 2) {
			L++;
		}
		else {
			L++, R--;
		}
	}

	cout << ans << endl;
	
	return 0;
}