#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) begin(v),end(v)
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
using ll = long long;
using pii = pair<int, int>;
constexpr ll INF = 1ll<<30;
constexpr ll longINF = 1ll<<60;
constexpr ll MOD = 1000000007;
constexpr bool debug = false;
//---------------------------------//

int main() {
	int N;
	cin >> N;
	vector<int> A(N);
	REP(i, N) scanf("%d", &A[i]), --A[i];
	
	vector<bool> done(N);
	int r = N;
	while (r > 0 && A[0] == A[r - 1]) --r;
	if (r == 0) { puts("0"); return 0; }
	bool ok = true;
	REP(i, r) {
		if (done[A[i]]) {
			if (i > 0 && A[i - 1] != A[i]) ok = false;
		}
		else done[A[i]] = true;
	}
	if (!ok) puts("-1");
	else if (r != N) puts("1");
	else puts("0");
}