#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

const int N = 100010;
int a[N];

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int n;
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        cin >> a[i];
    }
    int d = 0;
    if (a[1] < a[2])
    {
        d = a[2] - a[1];
    }
    else
    {
        d = a[2] - a[1] + n;
    }
    bool f = false;
    for (int i = 2; i <= n; i++)
    {
        int tmp = 0;
        if (a[i-1] < a[i])
        {
            tmp = a[i] - a[i-1];
        }
        else
        {
            tmp = a[i] - a[i-1] + n;
        }
        if(tmp != d){
            f = true;
            break;
        }
    }
    if(f) cout << -1 << endl;
    else{
        cout << 2*d - 1 << endl;
    }
    //system("pause");
    return 0;
}