#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1e5 + 10;
int a[MAXN];
int main() {
  int n;
  cin >> n;
  for (int i = 0; i < n; ++i) cin >> a[i];
  int k = 0;
  while (a[k] <= a[(k + 1) % n] && k < n) ++k;
  k += (k == n ? -n : 1);
  for (int i = 0; i < n - 1; ++i)
    if (a[(k + i) % n] > a[(k + i + 1) % n]) return cout << "-1", 0;
  cout << (n - k) % n;
  return 0;
}
