#include <bits/stdc++.h>
using namespace std;
int n, sum, idx = -1;
int main() {
  cin >> n;
  vector<int> v(n);
  for (int i = 0; i < n; i++) scanf("%d", &v[i]);
  for (int i = 0; i < n - 1; i++)
    if (v[i] > v[i + 1]) {
      idx = i;
      break;
    }
  if (idx == -1) {
    puts("0");
    return 0;
  } else {
    for (int i = n - 1; i > idx; i--) {
      if (i == n - 1) {
        if (v[i] > v[idx] || v[i] > v[0]) {
          printf("-1\n");
          return 0;
        } else
          sum++;
      } else {
        if (v[i] > v[idx] || v[i] > v[i + 1]) {
          printf("-1\n");
          return 0;
        } else
          sum++;
      }
    }
  }
  cout << sum << endl;
  return 0;
}
