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;
const long long int mod = 1e9 + 7;
const double pie = 3.14159265358979323846;
const char nl = '\n';
template <typename Type>
istream &operator>>(istream &in, vector<Type> &v) {
  long long int n = v.size();
  for (int i = 0; i < n; i++) in >> v[i];
  return in;
}
template <typename Type>
ostream &operator<<(ostream &out, vector<Type> &v) {
  for (auto value : v) out << value << " ";
  out << endl;
  return out;
}
void solve() {
  long long int n;
  cin >> n;
  vector<long long int> a(n);
  cin >> a;
  long long int i = 1;
  long long int k = a[0];
  while (i < n and a[i] >= a[i - 1]) ++i;
  if (i == n)
    cout << "0\n";
  else {
    long long int k = i;
    ++i;
    while (i < n and a[i] >= a[i - 1]) ++i;
    if (i == n and a[n - 1] <= a[0])
      cout << n - k << nl;
    else
      cout << "-1\n";
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int t;
  t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
