#include <bits/stdc++.h>
using namespace std;
#define int long long

signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int n;
    cin >> n;
    vector<int> a(n);
    for (int i = 0; i < n; i++)
        cin >> a[i];
    int cha = (a[1] - a[0] + n) % n;
    for (int i = 1; i < n; i++)
    {
        if ((a[i - 1] + cha) % n != a[i])
        {
            cout << "-1" << endl;
            return 0;
        }
    }
    if (cha == 0)
    {
        if (cha == 0 && a[0] == 0)
        {
            cout << 0 << endl;
            return 0;
        }
        else
            cout << n + 1 << endl;
        return 0;
    }
    if (a[0] == 0)
    {
        cout << cha << endl;
    }
    else
    {
        cout << cha + 1 << endl;
    }
}