#include <bits/stdc++.h>
#define int long long
#define endl "\n"

using namespace std;
typedef pair<int, int> PII;

const int N = 2e5 + 10;
int a[N];

void solve()
{
	int n;
	cin >> n;
	int chazhi = 0;
	bool ok = true;
	for (int i = 1; i <= n; i++)
	{
		cin >> a[i];
		if (i > 1)
		{
			if (a[i] >= a[i - 1])
			{
				if (!chazhi) chazhi = a[i] - a[i - 1];
				else
				{
					if (chazhi != a[i] - a[i - 1]) ok = false;
				}
			}
			else
			{
				if (!chazhi) chazhi = a[i] + n - a[i - 1];
				else
				{
					if (chazhi != a[i]  + n - a[i - 1]) ok = false;
				}
			}
		}
	}
	if (!ok)
	{
		cout << -1 << endl;
		return ;
	}
	cout << a[2] - a[1];
}

signed main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);

	int tt = 1;
//	cin>>tt;
	while (tt--) solve();
	return 0;
}