#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define endl '\n'
#define ull unsigned long long
using PII = pair<int, int>;
using ld = long double;
const int N = 1e5 + 5, M = 998244353;
 
typedef __int128 lll;
istream &operator>>(istream &cin, lll &x) { x=0; static string s; cin>>s; for (char c:s) x=x*10+(c-'0'); return cin; }
ostream &operator<<(ostream &cout, lll x) { static char s[60]; int tp=1; s[0]='0'+(x%10); while (x/=10) s[tp++]='0'+(x%10); while (tp--) cout<<s[tp]; return cout; }
 
mt19937_64 rng{ chrono::steady_clock::now().time_since_epoch().count() };

template <typename T>
int Min_show(T& A) {
    int i = 0, j = 1, k = 0;
    int n = A.size();
    while (i < n && j < n && k < n) {
        if (A[(i + k) % n] == A[(j + k) % n])k++;
        else {
            if (A[(i + k) % n] > A[(j + k) % n]) i += k + 1;
            else j += k + 1;
            if (i == j)i++;
            k = 0;
        }
    }
    return min(i, j);
}
void solve() {
    int n;
    cin >> n;
    vector<int>a(n);
    for (int i = 0;i < n;i++) {
        cin >> a[i];
    }
    ll d = (a[1] - a[0] + n) % n;
    for (int i = 2;i < n;i++) {
        if (d != (a[i] - a[i - 1] + n) % n) {
            cout << -1;
            return;
        }
    }
    vector<int>b(n);
    for (int i = 0;i < n;i++) {
        b[i] = i * d % n;
    }
    if (a == b) {
        cout << d;
        return;
    }
    int x = Min_show(a), y = Min_show(b);
    for (int i = 0;i < n;i++) {
        if (a[(i + x) % n] != b[(i + y) % n]) {
            cout << -1;
            return;
        }
    }
    cout << d + (a[0] != 0);
}
 
signed main() {
    ios::sync_with_stdio(0), cin.tie(0);
    int tt = 1;
    // cout << fixed << setprecision(5);
    // cin >> tt;
    while (tt--)solve();
}