#include <algorithm>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
using namespace std;
#define rep(i, n) for(int(i) = 0; (i) < (n); (i)++)
#define FOR(i, m, n) for(int(i) = (m); (i) < (n); (i)++)
#define All(v) (v).begin(), (v).end()
#define pb push_back
#define MP(a, b) make_pair((a), (b))
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
const int INF = 1 << 30;
const ll LINF = 1LL << 60;
const int MOD = 1e9 + 7;

int main() {
    int N;
    cin >> N;
    vector<int> A(N);
    rep(i, N) cin >> A[i];
    vector<int> B;
    int now = -1;
    rep(i, N) {
        if(now != A[i])
            B.pb(A[i]);
        now = A[i];
    }
    map<int, int> mp;
    rep(i, B.size()) { mp[B[i]]++; }
    int ok = 2;
    int cnt = 0;
    for(auto x : mp) {
        if(x.second == 2) {
            if(cnt == 0) {
                ok = 1;
            } else {
                ok = 0;
            }
            cnt++;
        } else if(x.second > 2) {
            ok = 0;
            break;
        }
    }

    if(ok == 2) {
        cout << 0 << endl;
        return 0;
    }

    if(ok == 1) {
        if(B.front() != B.back())
            ok = 0;
    }
    if(ok) {
        cout << 1 << endl;
    } else {
        cout << -1 << endl;
    }
    return 0;
}