#include <bits/stdc++.h>

using namespace std;

using ll = long long;

constexpr char newl = '\n';

bool judge(int n, const vector<int>& v) {
    vector<int> used(n, false);
    used[v[0]] = true;
    for (int i = 1; i < n; ++i) {
        if (v[i] == v[i - 1]) continue;
        if (used[v[i]]) return false;
        used[v[i]] = true;
    }
    return true;
}

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

    int n;
    cin >> n;

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

    int ans = 0;
    if (!judge(n, a)) {
        for (int i = 1; i < n; ++i) {
            if (a[i] == a[i - 1]) continue;
            rotate(a.begin(), a.begin() + i, a.end());
            break;
        }

        ans = (judge(n, a) ? 1 : -1);
    }
    cout << ans << newl;

    return 0;
}