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

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

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 ;
	}
	if(a[1] == 0) 
	{
		cout << a[2] << "\n";
		return;
	}
	cout << 3 << "\n";
}

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

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

answer.code: In function ‘void solve()’:
answer.code:16:24: error: ‘a’ was not declared in this scope
   16 |                 cin >> a[i];
      |                        ^
answer.code:42:12: error: ‘a’ was not declared in this scope
   42 |         if(a[1] == 0)
      |            ^