#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];
		if (a[i] == 0)
		{
			mk = i;
		}
	}
	if (n == 2 && a[0] == 1 && a[1] == 1)
	{
		cout << 3 << "\n";
		return 0;
	}
	for (int i = 0; i < n; i++)
	{
		if (a[i] < a[(i + 1)%n])
		{
			sum = a[(i + 1)%n] - a[i];
			break;
		}
	}
	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 = 0;
	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;
}
//#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 ans[N], ww[10000500];
//int main() {
//	IOS;
//	cin >> n;
//	ll s1 = 1;
//	for (ll i = 1; i <= n; i++) {
//		s1 = s1 * i % mod;
//		ww[i] = s1 % mod;
//	}
//	ans[1] = 1, ans[2] = 1, ans[3] = 3, ans[4] = 10;
//	if (n <= 4) cout << ans[n] % mod;
//	else {
//		ll sum = (n - 2) * ww[n - 2] + (n - 3) * ww[n - 2] - 2 + (n - 4) * ww[n - 2] - 6 * (n - 4) * ww[n - 4];
//		sum %= mod;
//		for (int i = 5; i < n - 1; i++) {
//			ll ssr = (2*(n-i-1)%mod+(n-i-1)*(n-i-2)/2%mod)*ww[n-3] % mod;
//			sum = (sum + ssr) % mod;
//		}
//		sum = (sum + ww[n - 1]) % mod;
//		cout << sum;
//	}
//	return 0;
//}