#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n, a, mx = 0, mn = 0;
  cin >> n;
  deque<int> unsorted, sorted;
  for (int i = 0; i < n; i++) {
    cin >> a;
    unsorted.push_back(a);
  }
  sorted = unsorted;
  sort(sorted.begin(), sorted.end());
  if (sorted == unsorted)
    cout << 0 << endl;
  else {
    int i = 0, cnt = 0;
    while (i < unsorted.size()) {
      if (unsorted[0] < unsorted[unsorted.size() - 1]) {
        break;
      }
      int x = unsorted.back();
      unsorted.pop_back();
      unsorted.push_front(x);
      cnt++;
      i++;
    }
    if (sorted == unsorted)
      cout << cnt << endl;
    else
      cout << -1 << endl;
  }
  return 0;
}
