#include <bits/stdc++.h>
using namespace std;
#define LL long long
#define pb push_back
#define x first
#define y second 
#define endl '\n'
const LL maxn = 4e05+7;
const LL N = 5e05+10;
const LL mod = 1e09+7;
const int inf = 0x3f3f3f3f;
const LL llinf = 5e18;
typedef pair<int,int>pl;
priority_queue<LL , vector<LL>, greater<LL> >mi;//小根堆
priority_queue<LL> ma;//大根堆
LL gcd(LL a, LL b){
	return b > 0 ? gcd(b , a % b) : a;
}

LL lcm(LL a , LL b){
	return a / gcd(a , b) * b;
}
int n , m;
vector<int>a(N , 0);
void init(int n){
	for(int i = 0 ; i <= n ; i ++){
		a[i] = 0;
	}
}
void solve() 
{
	cin >> n;
	for(int i = 0 ; i < n ; i ++){
		cin >> a[i];
	}	
	a[n] = a[0];
	for(int i = 1 ; i <= n ; i ++){
		if(a[i] < a[i - 1]){
			int k = (a[i - 1] - a[i] - 1 + n) / n;
			a[i] += k * n; 
		}
		if(a[i] - a[i - 1] != a[1] - a[0]){
			cout << -1;
			return;
		}
	}
	cout <<(a[1] - a[0]) * 2 - 1;
}            
int main() 
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    cout.precision(10);
    int t=1;
	//cin>>t;
    while(t--)
    {
    	solve();
    }
    return 0;
}