#include <bits/stdc++.h>
#define ll long long
const ll mod = 998244353;
using namespace std;
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0), cout.tie(0);
  int n;
  cin >> n;
  vector<int> a(n + 1);
  int zero = 0;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    if (!a[i])
      zero++;
  }
  int lst = a[1] - a[0];
  for (int i = 1; i < n - 1; i++) {
    if (a[i + 1] != (a[i] + lst + n) % n) {
      cout << -1 << '\n';
      return 0;
    }
  }
  ll one = 0, two = 0;
  int ans = 0;
  while (1) {
    if (a[0] != 0) {
      ans++;
      a[1] -= (a[0] + 1) % n;
      a[1] += n;
      a[1] %= n;
      a[0] = 0;
    } else if (a[1] != 0) {
      ans++;
      a[0] -= (a[1] - 1 + n) % n;
      a[0] += n;
      a[0] %= n;
      a[1] = 0;
    } else {
      break;
    }
  }

  if (n & 1 && ans)
    ans++;
  cout << ans;
  return 0;
}