#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, min, cur, prev, res = 0;
  cin >> n >> min;
  bool p = 0, f = 1;
  prev = min;
  int i = 0;
  for (; i < n - 1; i++) {
    cin >> cur;
    if (cur < prev) {
      p = 1;
      i++;
      break;
    } else
      prev = cur;
  }
  if (!p) {
    cout << 0 << endl;
    return 0;
  }
  if (cur > min)
    f = 0;
  else {
    prev = cur;
    res++;
  }
  for (; i < n - 1 && f; i++) {
    cin >> cur;
    if (cur < prev || cur > min)
      f = 0;
    else {
      prev = cur;
      res++;
    }
  }
  if (f)
    cout << res << endl;
  else
    cout << -1 << endl;
  return 0;
}
