#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int n, i, j, k = 0, l;
  cin >> n;
  vector<long long int> numbers(n), duplicate;
  for (i = 0; i < n; i++) cin >> numbers.at(i);
  duplicate = numbers;
  sort(duplicate.begin(), duplicate.end());
  if (duplicate == numbers) {
    cout << "0";
    return 0;
  }
  for (i = 1; i < n; i++) {
    if (numbers.at(i) < numbers.at(i - 1)) break;
  }
  if (numbers.at(i) > numbers.at(0)) {
    cout << "-1";
    return 0;
  }
  for (j = i + 1; j < n; j++) {
    if (numbers.at(j) < numbers.at(j - 1) || numbers.at(j) > numbers.at(0)) {
      k = -1;
      break;
    }
  }
  if (k == 0)
    cout << n - i;
  else
    cout << "-1";
  return 0;
}
