#include <iostream>
using namespace std;

int main ()
{
	int n;
	cin>>n;
	int arr[100005];
	int dem=0;
	int VT=-1;
	for (int i=1; i<=n; i++)
	{
		cin>>arr[i];
		if (i>1 && arr[i]<arr[i-1])
		{
			dem++;
			VT=i;
		}
	}
	if (dem>1) cout<<"-1";
	else if (dem==0) cout<<"0";
	else if (dem==1)
	{
		if (arr[1]>=arr[n])
		{
			cout<<(n-VT+1);
		}
		else cout<<"-1";
	}
	return 0;
}