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.*;
import java.math.BigInteger;
import java.util.*;
import static java.lang.Math.PI;
import static java.lang.System.in;
import static java.lang.System.out;

public class B
{
    public static void main(String [] args) throws Exception
    {
        Foster sc = new Foster();
        PrintWriter p = new PrintWriter(out);
        int n = sc.nextInt();
        int a[] = sc.intArray(n);
        int idx = 0;
        for(int i = 0; i < n-1; i++)
        {
            if(a[i] > a[i+1])
            {
                idx = i+1;
            }
        }
        boolean possible = true;
        ArrayList<Integer> arr = new ArrayList<>();
        for(int i = idx; i < n; i++)
        {
            arr.add(a[i]);
        }
        for(int i = 0; i < idx; i++)
        {
            arr.add(a[i]);
        }
        for(int i = 0; i < n-1; i++)
        {
            if(arr.get(i) > arr.get(i+1))
            {
                possible = false;
                break;
            }
        }
        if(idx == 0)
            p.println("0");
        else
            p.println(possible ? n-idx : "-1");
        p.close();
    }

/*
    Move to top!!
*/
    static class Foster 
    {
        BufferedReader br = new BufferedReader(new InputStreamReader(in));
        StringTokenizer st = new StringTokenizer("");
        String next() 
        {
            while (!st.hasMoreTokens())
                try 
                {
                    st = new StringTokenizer(br.readLine());
                } 
                catch (IOException e) 
                {
                    e.printStackTrace();
                }
            return st.nextToken();
        }
        int nextInt() 
        {
            return Integer.parseInt(next());
        }
        long nextLong() 
        {
            return Long.parseLong(next());
        }
        double nextDouble() 
        {
            return Double.parseDouble(next());
        }
        int[] intArray(int n)                   // Check indices
        {
            int arr[] = new int[n];
            for(int i = 0; i < n; i++)
            {
                arr[i] = nextInt();
            }
            return arr;
        }
        long[] longArray(int n)                 // Check indices
        {
            long arr[] = new long[n];
            for(int i = 0; i < n; i++)
            {
                arr[i] = nextLong();
            }
            return arr;
        }
        int[] getBits(int n)                    //in Reverse Order
        {
            int a[] = new int[31];
            for(int i = 0; i < 31; i++)
            {
                if(((1<<i) & n) != 0)
                    a[i] = 1;
            }
            return a;
        }
        static long pow(long... a)
        {
            long mod = Long.MAX_VALUE;
            if(a.length == 3)   mod = a[2];
            long res = 1;
            while(a[1] > 0)
            {
                if((a[1] & 1) == 1)
                    res = (res * a[0]) % mod;
                a[1] /= 2;
                a[0] = (a[0] * a[0]) % mod;
            }
            return res;
        }
        void print(Object... o) 
        {
            for(Object next : o)
            {
                System.out.print(next + " ");
            }
        }
        void println(Object... o) 
        {
            for(Object next : o)
            {
                System.out.print(next + " ");
            }
            System.out.println();
        }
        void watch(Object...a) throws Exception 
        {
            int i = 1;
            for (Object o: a) 
            {
                boolean found = false;
                if (o.getClass().isArray()) 
                {
                    String type = o.getClass().getName().toString();
                    switch (type) 
                    {
                        case "[I":
                        {
                            int[] test = (int[]) o;
                            println(i + " : " + Arrays.toString(test));
                            break;
                        }
                        case "[[I":
                        {
                            int[][] obj = (int[][]) o;

                            println(i + " : " + Arrays.deepToString(obj));
                            break;
                        }
                        case "[J":
                        {

                            long[] obj = (long[]) o;
                            println(i + " : " + Arrays.toString(obj));
                            break;
                        }
                        case "[[J":
                        {

                            long[][] obj = (long[][]) o;
                            println(i + " : " + Arrays.deepToString(obj));
                            break;
                        }
                        case "[D":
                        {

                            double[] obj = (double[]) o;
                            println(i + " : " + Arrays.toString(obj));
                            break;
                        }
                        case "[[D":
                        {

                            double[][] obj = (double[][]) o;
                            println(i + " : " + Arrays.deepToString(obj));
                            break;
                        }
                        case "[Ljava.lang.String":
                        {

                            String[] obj = (String[]) o;
                            println(i + " : " + Arrays.toString(obj));
                            break;
                        }
                        case "[[Ljava.lang.String":
                        {

                            String[][] obj = (String[][]) o;
                            println(i + " : " + Arrays.deepToString(obj));
                            break;
                        }
                        case "[C":
                        {
                            char[] obj = (char[]) o;
                            println(i + " : " + Arrays.toString(obj));
                            break;
                        }
                        case "[[C":
                        {

                            char[][] obj = (char[][]) o;
                            println(i + " : " + Arrays.deepToString(obj));
                            break;
                        }
                        default:
                        {
                            println(i + " : type not identified");
                            break;
                        }
                    }
                    found = true;
                }
                if (o.getClass() == ArrayList.class) 
                {
                    println(i + " : LIST = " + o);
                    found = true;
                }
                if (o.getClass() == TreeSet.class) 
                {
                    println(i + " : SET = " + o);
                    found = true;
                }
                if (o.getClass() == TreeMap.class) 
                {
                    println(i + " : MAP = " + o);
                    found = true;
                }
                if (o.getClass() == HashMap.class) 
                {
                    println(i + " : MAP = " + o);
                    found = true;
                }
                if (o.getClass() == LinkedList.class) 
                {
                    println(i + " : LIST = " + o);
                    found = true;
                }
                if (o.getClass() == PriorityQueue.class) 
                {
                    println(i + " : PQ = " + o);
                    found = true;
                }
                if (!found) 
                {
                    println(i + " = " + o);
                }
                i++;
            }
        }
    }
}