#include<bits/stdc++.h>
using ll = long long;
using namespace std;    
const int N = 1e5 + 10;
const int mod = 998244353;


void syncoff() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);cout.tie(nullptr);
}

int a[N], b[N], c[N];
map <int, int> cnt;

void solve() {
    int n, s = -1, s1 = -1;
    cin >> n;
    for (int i = 1;i <= n;i++) {
        cin >> a[i];
        if (a[i] == 0 && s == -1)s = i;
        if (a[i] == 1 && s1 == -1)s1 = i;
        cnt[a[i]]++;
    }

    bool flag = 1;
    for (int i = 1;i <= n;i++) {
        if (a[i] != 0) { flag = 0;break; }
    }
    if (flag) { cout << 0 << endl; return; }

    flag = 1;
    for (int i = 1;i <= n;i++) {
        if (a[i] != i - 1) { flag = 0;break; }
    }
    if (flag) { cout << 1 << endl; return; }

    for (int i = 1;i <= n;i++) {
        b[i] = ((i - 1) * 2) % n;
    }
    flag = 1;
    for (int i = 1;i <= n;i++) {
        if (a[i] != b[i]) { flag = 0;break; }
    }
    if (flag) { cout << 2 << endl; return; }


    if (n & 1) {
        flag = 1;
        for (int i = 0;i < n;i++) {
            if (cnt[i] != 1) { flag = 0; break; }
        }
        if (!flag) { cout << -1 << endl; return; }

        int cnt = 0;
        bool f1 = 1;
        for (int i = s;i <= n;i++) {
            if (a[i] == cnt) {
                cnt++;
            }
            else f1 = 0;
        }

        for (int i = 1;i < s;i++) {
            if (a[i] == cnt) {
                cnt++;
            }
            else f1 = 0;
        }

        if (f1) { cout << 2 << endl; return; }

        f1 = 1;
        cnt = 0;
        for (int i = s;i <= n;i++) {
            if (a[i] == cnt) {
                cnt += 2;
                cnt %= n;
            }
            else f1 = 0;
        }

        for (int i = 1;i < s;i++) {
            if (a[i] == cnt) {
                cnt += 2;
                cnt %= n;
            }
            else f1 = 0;
        }

        if (f1) { cout << 3 << endl; return; }
        cout << -1 << endl;
    }
    else {
        int cnt = 1;

        bool f1 = 1;
        cnt = 0;
        for (int i = s;i <= n;i++) {
            if (a[i] == cnt) {
                cnt += 2;
                cnt %= n;
            }
            else f1 = 0;
        }
        for (int i = 1;i < s;i++) {
            if (a[i] == cnt) {
                cnt += 2;
                cnt %= n;
            }
            else f1 = 0;
        }
        if (f1 && s == 1) { cout << 2 << endl; return; }
        if (f1 && s != 1) { cout << 3 << endl; return; }


        f1 = 1;
        cnt = 1;
        for (int i = s1;i <= n;i++) {
            if (a[i] == cnt) {
                cnt += 2;
                cnt %= n;
            }
            else f1 = 0;
        }
        for (int i = 1;i < s;i++) {
            if (a[i] == cnt) {
                cnt += 2;
                cnt %= n;
            }
            else f1 = 0;
        }
        if (f1) { cout << 3 << endl; return; }
        cout << -1 << endl;

    }

}

signed main() {
    syncoff();
    ll _ = 1;
    //cin >> _;
    while (_--) {
        solve();
    }
    return 0;
}