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
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;


public class R259 {
	public static void main(String[] args) throws NumberFormatException, IOException {
		BufferedReader f = new BufferedReader(new InputStreamReader(System.in));
		PrintWriter out = new PrintWriter(System.out);
		
		int n = Integer.parseInt(f.readLine());
		String[] split = f.readLine().split(" ");
		int[] a = new int[n];
		for(int i = 0; i < n; i++) a[i] = Integer.parseInt(split[i]);
		boolean shiftSort = false;
		int cnt = 0, smallestNum = a[0], largestNum=a[n-1];
		for(int i = 0; i < n-1; i++) {
			if(a[i] > a[i+1] && !shiftSort) {
				shiftSort=true;
				cnt=i;
			}
			else if(a[i] > a[i+1] && shiftSort) {
				out.println(-1);
				out.flush();
				return;
			}
		}
		if(!shiftSort) {
			out.println(0);
		} else if(largestNum<=smallestNum){
			out.println(Math.max(0,n-cnt-1));
		} else {
			out.println(-1);
		}
		out.close();
	}
}
