#pragma GCC optimize("O3", "unroll-loops")

#include <bits/stdc++.h>

using namespace std;

#define ll long long

int main(void){
    int N; cin >> N;
    vector<int> A(N);
    for (int i = 0; i < N; ++i)
        cin >> A[i];
    // 全部同じ色か判定
    bool all_same = true;
    for (int i = 1; i < N; ++i)
        all_same &= A[i] == A[i - 1];
    if (all_same){
        cout << 0 << endl;
        return 0;
    }
    // 区切れ目を求める
    int idx = 0;
    while (A[idx] == A[(idx - 1 + N) % N])
        idx = (idx - 1 + N) % N;
    // 答えを求める
    map<int, int> appered;
    appered[A[idx]] = 1;
    for (int i = 1; i < N; ++i){
        if (A[(idx + i) % N] == A[(idx + i - 1) % N])
            continue;
        if (appered[A[(idx + i) % N]]){
            cout << -1 << endl;
            return 0;
        }
        else appered[A[(idx + i) % N]] = 1;
    }
    if (idx == 0) cout << 0 << endl;
    else cout << 1 << endl;
    return 0;
}