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;
string IntToString(int Number) {
  std::ostringstream ss;
  ss << Number;
  return ss.str();
}
int evaluteOld(vector<int> numbers, string line) {
  int weight = 0;
  for (int i = 0; i < line.length(); i++) {
    weight += (i + 1) * numbers[line[i] - 'a'];
  }
  return weight;
}
int evaluteNew(int k, int max, int length) {
  int weight = 0;
  for (int i = length; i < k + length; i++) {
    weight += (i + 1) * max;
  }
  return weight;
}
int main() {
  string line;
  getline(cin, line);
  int n;
  int start = n;
  int end;
  int problems = 0;
  stringstream ss(line);
  ss >> n;
  getline(cin, line);
  stringstream ssNumbers(line);
  int number;
  ssNumbers >> number;
  int prev = number;
  start = number;
  int current;
  int length = n;
  int index;
  n--;
  while (n != 0) {
    ssNumbers >> number;
    current = number;
    if (current < prev) {
      index = length - n;
      problems++;
    }
    if (problems > 1) {
      cout << -1;
      return 0;
    }
    prev = number;
    n--;
  }
  end = number;
  if (problems == 0) {
    cout << 0 << endl;
  } else if (problems == 1 && end > start) {
    cout << -1;
  } else {
    cout << length - index;
  }
  return 0;
}
