#include <bits/stdc++.h>
using namespace std;
bool check(vector<long long int> v) {
  long long int k = v.size();
  for (long long int i = 0; i < k - 1; i++) {
    if (v[i] > v[i + 1]) return 0;
  }
  return 1;
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int n;
  cin >> n;
  long long int p[n];
  for (long long int i = 0; i < n; i++) cin >> p[i];
  vector<long long int> v;
  long long int index = -1;
  for (long long int i = 0; i < n - 1; i++) {
    if (p[i] > p[i + 1]) {
      index = i + 1;
    }
  }
  if (index == -1) {
    cout << 0 << endl;
    return 0;
  }
  for (long long int i = index; i < n; i++) {
    v.push_back(p[i]);
  }
  for (long long int i = 0; i < index; i++) {
    v.push_back(p[i]);
  }
  if (check(v))
    cout << n - index << endl;
  else
    cout << -1 << endl;
  return 0;
}
