#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;
using ll = long long;

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

    int n;
    cin >> n;

    vector<int> a(n), b(n, -1);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
        a[i]--;
    }

    n = unique(a.begin(), a.end()) - a.begin();

    int r = 0;
    for (int i = 0; i < n; i++) {
        int j = a[i];
        if (b[j] >= 0) {
            r = (b[j] == 0 && i == n - 1) ? 1 : -1;
            break;
        }
        b[j] = i;
    }

    cout << r << endl;

    return 0;
}