#include<bits/stdc++.h>
using namespace std;

const int N = 1000;

void solve() {
    int n; cin >> n;
    vector<int> a(n + 10, 0), p(n + 10);
    for (int i = 1; i <= n; i ++) cin >> a[i], p[i] = i - 1;

    int f = 0;
    for (int i = 1; i <= n; i ++) {
        if (a[i]) f = 1;
    }
    if (!f) {
        cout << 0 << '\n';
        return;
    }

    int add;
    if (a[1] < a[2]) add = a[2] - a[1];
    else add = a[2] + n - a[1];
    for (int i = 3; i <= n; i ++) {
        if (a[i] != (a[i - 1] + add) % n) {
            cout << -1 << '\n';
            return;
        }
    }
    if (a[1] == 0) cout << add << '\n';
    else cout << add + 1 << '\n';
}

signed main() {
    int t = 1;
    while (t --) solve();
}