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.util.*;
import java.lang.*;

public class Main { 
    public static void main(String[] args) { 
        InputStream input = System.in;
        OutputStream output = System.out;
        InputReader in = new InputReader(input);
        PrintWriter out = new PrintWriter(output);
        Solution s = new Solution();
        s.solve(1, in, out);
        out.close();
    }

    static class Solution { 
        
        double EPS = 0.0000001;
        public void solve(int cs, InputReader in, PrintWriter out) {
            int n = in.nextInt();
            int[] arr = new int[n];
            arr[0] = in.nextInt();
            int dec = 0, idx = 0;
            for (int i = 1 ; i < n; i++) {
                arr[i] = in.nextInt();
                if (arr[i] < arr[i-1]) {
                    dec++;
                    idx = i;
                }
            }
            if (dec > 0 && arr[n-1] > arr[0])
                dec++;
            if (dec == 0)
                out.println(0);
            else if (dec == 1)
                out.println(n-idx);
            else
                out.println(-1);


        }
    }

    static class InputReader { 
        BufferedReader br;
        StringTokenizer st;
        
        public InputReader(InputStream i) {
            br = new BufferedReader(new InputStreamReader(i), 32768);
            st = null;
        }

        public InputReader(FileReader s) {
            br = new BufferedReader(s);
            st = null;
        }

        public String next() {
            while (st == null || !st.hasMoreTokens()) {
                try { 
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return st.nextToken();
        }

        public int nextInt() { 
            return Integer.parseInt(next());
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }

        public String nextLine() { 
            try { 
                return br.readLine();
            } catch (IOException e) { 
                throw new RuntimeException(e);
            }
        }
    }
    static class Reader {
        final private int BUFFER_SIZE = 1 << 16;
        private DataInputStream din;
        private byte[] buffer;
        private int bufferPointer, bytesRead;
        public Reader(){
            din=new DataInputStream(System.in);
            buffer=new byte[BUFFER_SIZE];
            bufferPointer=bytesRead=0;
        }

        public Reader(String file_name) throws IOException{
            din=new DataInputStream(new FileInputStream(file_name));
            buffer=new byte[BUFFER_SIZE];
            bufferPointer=bytesRead=0;
        }

        public String readLine() throws IOException{
            byte[] buf=new byte[64]; // line length
            int cnt=0,c;
            while((c=read())!=-1){
                if(c=='\n')break;
                buf[cnt++]=(byte)c;
            }
            return new String(buf,0,cnt);
        }

        public int nextInt() throws IOException{
            int ret=0;byte c=read();
            while(c<=' ')c=read();
            boolean neg=(c=='-');
            if(neg)c=read();
            do{ret=ret*10+c-'0';}while((c=read())>='0'&&c<='9');
            if(neg)return -ret;
            return ret;
        }

        public long nextLong() throws IOException{
            long ret=0;byte c=read();
            while(c<=' ')c=read();
            boolean neg=(c=='-');
            if(neg)c=read();
            do{ret=ret*10+c-'0';}while((c=read())>='0'&&c<='9');
            if(neg)return -ret;
            return ret;
        }

        public double nextDouble() throws IOException{
            double ret=0,div=1;byte c=read();
            while(c<=' ')c=read();
            boolean neg=(c=='-');
            if(neg)c = read();
            do {ret=ret*10+c-'0';}while((c=read())>='0'&&c<='9');
            if(c=='.')while((c=read())>='0'&&c<='9')
                ret+=(c-'0')/(div*=10);
            if(neg)return -ret;
            return ret;
        }

        private void fillBuffer() throws IOException{
            bytesRead=din.read(buffer,bufferPointer=0,BUFFER_SIZE);
            if(bytesRead==-1)buffer[0]=-1;
        }

        private byte read() throws IOException{
            if(bufferPointer==bytesRead)fillBuffer();
            return buffer[bufferPointer++];
        }

        public void close() throws IOException{
            if(din==null) return;
            din.close();
        }
    }
}


