#include <bits/stdc++.h>
using namespace std;
const int Mod = 1e9 + 7;
const int inf = 1 << 31;
map<string, int> Map;
map<string, int>::iterator it;
const int N = 100005;
int a[N];
int main() {
  int n;
  int pos;
  int i, j, k;
  while (cin >> n) {
    int sum = 0;
    for (i = 0; i < n; i++) cin >> a[i];
    for (i = 1; i < n; i++) {
      if (a[i] < a[i - 1]) {
        sum++;
        pos = n - i;
      }
    }
    if (sum == 0)
      cout << 0 << endl;
    else if (sum > 1)
      cout << -1 << endl;
    else {
      if (a[n - 1] <= a[0])
        cout << pos << endl;
      else
        cout << -1 << endl;
    }
  }
  return 0;
}
