#include <bits/stdc++.h>
using namespace std;
#define N 200005
#define int long long
#define endl '\n'
int ans = 0, n, gc, a[N];
signed main()
{
    cin >> n;
    for (int i = 1; i <= n; i++)
        cin >> a[i];
    gc = (a[2] - a[1] + n - 1) % n + 1; // 公差
    for (int i = 2; i <= n; i++)
        if ((a[i % n + 1] - a[i] + n - 1) % n + 1 != gc)
        {
            cout << -1;
            // cout << i;
            return 0;
        }
    cout << ((a[1] == 0) + gc);
    return 0;
}