One day, Twilight Sparkle is interested in how to sort a sequence of integers a1, a2, ..., an in non-decreasing order. Being a young unicorn, the only operation she can perform is a unit shift. That is, she can move the last element of the sequence to its beginning:

a1, a2, ..., an → an, a1, a2, ..., an - 1.

Help Twilight Sparkle to calculate: what is the minimum number of operations that she needs to sort the sequence?

Input

The first line contains an integer n (2 ≤ n ≤ 105). The second line contains n integer numbers a1, a2, ..., an (1 ≤ ai ≤ 105).

Output

If it's impossible to sort the sequence output -1. Otherwise output the minimum number of operations Twilight Sparkle needs to sort it.

Examples

Input

2
2 1


Output

1


Input

3
1 3 2


Output

-1


Input

2
1 2


Output

0
#include <bits/stdc++.h>
using namespace std;
int toInt(string s) {
  istringstream iss(s);
  int res;
  if (!(iss >> res)) res = -1;
  return res;
}
string toString(int a) {
  ostringstream oss;
  oss << a;
  return oss.str();
}
vector<string> getTokens(string s, char delim) {
  vector<string> tokens;
  istringstream iss(s);
  string temp;
  while (getline(iss, temp, delim)) {
    tokens.push_back(temp);
  }
  return tokens;
}
class Codeforces {
 public:
  int task(vector<int> a) {
    int j = -1;
    for (int i = 0; i < a.size(); i++) {
      if (i + 1 < a.size() && a[i + 1] < a[i]) {
        j = i + 1;
        break;
      }
    }
    if (j == -1) return 0;
    for (int i = j; i < a.size(); i++) {
      if (i + 1 < a.size() && a[i + 1] < a[i]) {
        return -1;
      }
    }
    if (a[a.size() - 1] <= a[0])
      return (a.size() - j);
    else
      return -1;
  }
};
int main() {
  Codeforces cf = Codeforces();
  int n;
  cin >> n;
  vector<int> a;
  a.resize(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  cout << cf.task(a) << "\n";
  return 0;
}
