#define _CRT_SECURE_NO_WARNINGS
////#pragma GCC optimize(1)
////#pragma GCC optimize(2)
////#pragma GCC optimize(3,"Ofast","inline")
#include<bits/stdc++.h>
using namespace std;
#define N 2000005 
#define M 1331 
#define PII pair<int,int>
#define PIII pair<pair<int,int>,int>  
#define VPII vector<PII>
#define VPIII vector<PIII>
#define endl "\n"
#define inf 0x3f3f3f3f3f3f  
#define ll long long
#define ull unsigned long long
const double eps = 1e-6;
#define IOS ios::sync_with_stdio(false),cin.tie(0),cout.tie(0)
#define mod  998244353
ll n, m,k;
ll a[N];
ll b[N];
int main()
{
	cin >> n;
	ll sum = 0;
	ll mk = 0;
	for (int i = 0; i < n; i++)
	{
		cin >> a[i];
	}
	for (int i = 0; i < n; i++)
	{
		if (a[i] == 0)
		{
			mk = i;
			break;
		}
	}
	ll cnt = 0;
	for (int i = 0; i < n; i++)
	{
		if (a[i] < a[(i + 1)%n])
		{
			sum = a[(i + 1)%n] - a[i];
			if (cnt == 0)
			{
				cnt == sum;
			}
			else
			{
				if (cnt != sum)
				{
					cout << "-1\n";
					return 0;
				}
			}
		}
	}
	for (ll i = 0; i < n; i++)
	{
		b[i] = sum * i;
		b[i] %= n;
	}
	int ans = 0;
	for (int i = 0; i < n; i++)
	{
		if (a[i] == 0)
		{
			ans++;
		}
	}
	if (ans == n)
	{
		cout << "0\n";
		return 0;
	}
	ans = 0;
	int as = mk;
	for (int i = 0; i < n; i++)
	{
		if (a[mk % n] == b[i])
		{
			ans++;
			mk++;
		}
		else
		{
			ans = -1;
			break;
		}
	}
	if (ans == -1)
	{
		cout << ans << "\n";
		return 0;
	}
	if (as == 0)
	{
		cout << sum << "\n";
	}
	else
	{
		cout << sum + 1 << "\n";
	}
	return 0;
}