#include <bits/stdc++.h>
using namespace std;
int main(int argc, char **argv) {
  list<int> vec1;
  list<int> vec2;
  int n;
  cin >> n;
  int auxn = n;
  int num;
  for (int i = 0; i < n; i++) {
    cin >> num;
    vec1.push_back(num);
  }
  vec2 = vec1;
  vec2.sort();
  list<int> aux;
  int auxnum;
  int cont = 0;
  int tam;
  for (int j = 0; j < n; j++) {
    if (vec1 != vec2) {
      aux.push_back(vec1.back());
      auxnum = vec1.back();
      vec1.pop_back();
      while (auxnum == vec1.back()) {
        aux.push_back(vec1.back());
        vec1.pop_back();
        n--;
      }
      tam = aux.size();
      for (int s = 0; s < tam; s++) {
        vec1.push_front(auxnum);
        aux.pop_back();
        cont++;
      }
    } else
      break;
  }
  if (cont == auxn)
    cout << -1 << endl;
  else
    cout << cont << endl;
  return 0;
}
