#include <bits/stdc++.h>
using namespace std;
//upper_bound	>
//lower_bound	>=
#define LL long long
#define ULL unsigned long long
#define PII pair<int,int>
#define YES cout<<"YES\n"
#define NO cout<<"NO\n"
#define PI acos(-1)
#define endl '\n'
#define ee cout<<'\n';
#define __lcm(a, b) a*b/__gcd(a,b)
#define fo(a,b,c) for(int a=b;a<=c;a++)
#define of(a,b,c) for(int a=b;a>=c;a--)
#define debug(x) cout<<#x<<' '<<x<<endl;
#define nep next_permutation
#define prep prev_permutation
#define g(a) a,vector<a>,greater<a>
#define all(a) a.begin(),a.end()
#define alls(a) a.begin()+1,a.end()
#define int long long
//const int p = 998244353, pp = 1000000007;
LL qmi(LL a,LL b,LL mod){LL res=1;while(b){if(b&1)res=res*a%mod;b>>=1;a=a*a%mod;}return res;	}
const int N = 2000010, M = 4000010;
int a[N];
int c[N];
void solve()
{
	int n;
	cin >> n;
	int idx = -1;
	for (int i = 0; i < n; i ++)
	{
		cin >> a[i];
		if (a[i] == 0) idx = i;
	}
	vector<int> b;
	for (int i = 0; i < n; i ++)
	{
		b.push_back(a[(i + idx) % n]);
	}
	int c = b[1] - b[0];
	for (int i = 1; i < n - 1; i ++)
	{
		if ((b[i] + c) % n == b[i + 1]) continue;
		else 
		{
			cout << -1;
			return;
		}
	}
	if (idx == 0) cout << c;
	else cout << c + 1;
	
}
signed main()
{
 	ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
	//int T; cin >> T; while (T --)
	solve();
}