It's been almost a week since Polycarp couldn't get rid of insomnia. And as you may already know, one week in Berland lasts k days!

When Polycarp went to a doctor with his problem, the doctor asked him about his sleeping schedule (more specifically, the average amount of hours of sleep per week). Luckily, Polycarp kept records of sleep times for the last n days. So now he has a sequence a1, a2, ..., an, where ai is the sleep time on the i-th day.

The number of records is so large that Polycarp is unable to calculate the average value by himself. Thus he is asking you to help him with the calculations. To get the average Polycarp is going to consider k consecutive days as a week. So there will be n - k + 1 weeks to take into consideration. For example, if k = 2, n = 3 and a = [3, 4, 7], then the result is <image>.

You should write a program which will calculate average sleep times of Polycarp over all weeks.

Input

The first line contains two integer numbers n and k (1 ≤ k ≤ n ≤ 2·105).

The second line contains n integer numbers a1, a2, ..., an (1 ≤ ai ≤ 105).

Output

Output average sleeping time over all weeks. 

The answer is considered to be correct if its absolute or relative error does not exceed 10 - 6. In particular, it is enough to output real number with at least 6 digits after the decimal point.

Examples

Input

3 2
3 4 7


Output

9.0000000000


Input

1 1
10


Output

10.0000000000


Input

8 2
1 2 4 100000 123 456 789 1


Output

28964.2857142857

Note

In the third example there are n - k + 1 = 7 weeks, so the answer is sums of all weeks divided by 7.
 // Importing all important imports
     
    import java.awt.Point;
    import static java.lang.System.in;
    import java.io.BufferedReader;
    import java.io.ByteArrayInputStream;
    import java.io.DataInputStream;
    import java.io.FileNotFoundException;
    import java.io.FileOutputStream;
    import java.io.IOException;
    import java.io.InputStream;
    import java.io.InputStreamReader;
    import java.io.OutputStream;
    import java.io.OutputStreamWriter;
    import java.io.PrintStream;
    import java.io.PrintWriter;
    import java.math.BigInteger;
    import java.math.BigDecimal;
    import java.math.MathContext;
    import java.math.RoundingMode;
    import java.nio.file.Files;
    import java.nio.file.Paths;
    import java.text.ParseException;
    import java.text.SimpleDateFormat;
    import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.BitSet;
    import java.util.Collections;
    import java.util.Comparator;
    import java.util.Date;
    import java.util.GregorianCalendar;
    import java.util.HashMap;
    import java.util.HashSet;
    import java.util.Hashtable;
    import java.util.InputMismatchException;
    import java.util.LinkedHashMap;
    import java.util.LinkedList;
    import java.util.List;
    import java.util.Map;
    import java.util.Stack;
    import java.util.Queue;
    import java.util.Random;
    import java.util.regex.Matcher;
    import java.util.regex.Pattern;
    import java.util.Set;
    import java.util.SortedSet;
    import java.util.StringTokenizer;
    import java.util.TreeMap;
    import java.util.TreeSet;
    import java.util.Vector;
     
     
     
    /**
    @author
    * 
    $$\      $$\            $$$$$$\            
    $$$\    $$$ |          $$  __$$\           
    $$$$\  $$$$ | $$$$$$$\ $$ /  \__|  
    $$\$$\$$ $$ |$$  _____|$$$$\      
    $$ \$$$  $$ |\$$$$$$\  $$  _|     
    $$ |\$  /$$ | \____$$\ $$ |      
    $$ | \_/ $$ |$$$$$$$  |$$ |      
    \__|     \__|\_______/ \__|       
    *
    * @Mustafa Filmwala
    * @filmwalams@gmail.com
    * https://github.com/filmwalams/JAVA-Functions
    * 
    * 
    */
     
	public class Main{
            
            static boolean testcase=false;
            static boolean debug_mode=false;
            static String input_file_name="";
            static String output_file_name="";
            
            //static String input_file_name="input.txt";
            //static String output_file_name="output.txt";
            
            public int mod=1000000007;
  
            public void solve(int tc) throws IOException           {   
            	int n = ni(),k = ni();
            	int arr[] = nia(n);
            	long sum = 0;
            	for(int i = 0 ;i<n;i++){
            		int mul = Math.min(Math.min(k,n-k+1),Math.min(i-0+1,n-i));
            		//out.println(mul);
            		sum += (long)mul*arr[i];
            	}
            	out.printf("%.6f\n",(double)sum/(double)(n-k+1)*1.0);
			}
            
     
            // END OF SOLUTION
            
            public void precompute(){}
            
            
            // INPUT OUTPUT TEMPLATE
            static String TESTCASES="";
            static PrintWriter out;
            static int Space;
            
            public static void main(String[] args) throws Exception{
                    // generateTestCases(1); 
                    long endTime,startTime=System.currentTimeMillis();
                    if(output_file_name.isEmpty())
    					out=new PrintWriter(new OutputStreamWriter(System.out));
                    else
    					out = new PrintWriter(new FileOutputStream("/home/filmwalams/Desktop/CONTEST/"+output_file_name));
                    //if(!input_file_name.isEmpty()){
    				//	mytestcases();
    				//}
                    InputStream myinp=TESTCASES.isEmpty() ? System.in : new ByteArrayInputStream(TESTCASES.getBytes());
                    init(myinp);
                    
                    Space=0;
                    Main program=new Main();
                    
                    program.precompute();
                    if(testcase==true){int test=ni();for(int tc=1;tc<=test;tc++)program.solve(tc);}
                    else program.solve(0);
                    
                    out.close();
                    endTime=System.currentTimeMillis();
                    //printTime("Time: "+(endTime-startTime)+"ms");
            }
            
            // Testcases reader from FILE. If file not found, input is taken from console.
            public static void mytestcases(){
    			
                try{
    				TESTCASES=new String(Files.readAllBytes(Paths.get("/home/filmwalams/Desktop/CONTEST/"+input_file_name)));
                }
                catch(Exception e){
    				TESTCASES="";
    			}
            }
            
            public static void generateTestCases(int count){
    			
                PrintStream console=System.out;
                try{
    				PrintStream out = new PrintStream(new FileOutputStream("/home/filmwalams/Downloads"));
                
    				System.setOut(out);
    				Random r=new Random();
    				// System.out.println(count);
    				for(int tc=0;tc<count;tc++){
    					   
    				}
    				System.setOut(console);
                }
                catch(FileNotFoundException fe){
    				System.setOut(console);
    			}
            }
              
              
            // Just to print time, not to use anywhere else
            public static void printTime(Object ...obj){System.err.println(Arrays.deepToString(obj));}
            static InputStream is;
            static BufferedReader brr;
            protected static byte[] inbuf = new byte[1024];
            static int lbuffer = 0, pbuffer = 0;
            public static void init(InputStream input) {is=input;brr=new BufferedReader(new InputStreamReader(input));}
            // Fundamental
            protected static int rb(){if(lbuffer == -1)throw new InputMismatchException();if(pbuffer >= lbuffer){pbuffer = 0;try { lbuffer = is.read(inbuf); } catch (IOException e) { throw new InputMismatchException(); }if(lbuffer <= 0)return -1;}return inbuf[pbuffer++];}       
            protected static String line() throws IOException{return brr.readLine();}   
            protected static boolean isSpace(int c) { return !(c >= 33 && c <= 126); }
            protected static int skip(){ int b; while((b = rb()) != -1 && isSpace(b)); return b; }
            // Inputs
            protected static double nd() throws IOException { return Double.parseDouble(ns()); }
            protected static char nc() { return (char)skip(); }
            protected static String ns() throws IOException{if(Space==1)return line();int b = skip();StringBuilder sb = new StringBuilder();while(!(isSpace(b))){ sb.appendCodePoint(b);b = rb();}return sb.toString();}
            protected static char[] nca(int n){char[] buf = new char[n];int b = skip(), p = 0;while(p < n && !(isSpace(b))){buf[p++] = (char)b;b = rb();} return n == p ? buf : Arrays.copyOf(buf, p);}
            protected static int[] nia(int n) throws IOException{int[] a = new int[n];for(int i = 0;i < n;i++)a[i] = ni();return a;}
            protected static int[][] niaindex(int n) throws IOException{int ea[][]=new int[n][2];for(int i=0;i<n;i++){ea[i][0]=ni();ea[i][1]=i;}return ea;}
            protected static long[] nla(int n) throws IOException{long[] a = new long[n];for(int i = 0;i < n;i++)a[i] = nl();return a;}
            protected static long[][] nlaindex(int n) throws IOException{long ea[][]=new long[n][2];for(int i=0;i<n;i++){ea[i][0]=ni();ea[i][1]=i;}return ea;}
            protected static String[] nsa(int n) throws IOException{String[] a = new String[n];for(int i = 0;i < n;i++)a[i] = ns();return a;    }
            protected static double[] nda(int n) throws IOException{double[] a = new double[n];for(int i = 0;i < n;i++)a[i] = nd();return a;}
            protected static int ni() throws IOException{if(Space==1)return Integer.parseInt(line());int num=0,b;boolean minus = false;while((b = rb())!= -1 && !((b>='0'&&b<='9') || b =='-'));if(b =='-'){minus=true;b = rb();    }while(true){if(b >= '0' && b <= '9'){num = num * 10 + (b - '0');}else{return minus ? -num : num;}b = rb();}}  
            protected static long nl() throws IOException{if(Space==1)return Long.parseLong(line());long num = 0;int b;boolean minus = false;while((b = rb()) != -1 && !((b >= '0' && b <= '9') || b == '-'));if(b == '-'){   minus = true;b = rb();}while(true){if(b >= '0' && b <= '9'){num = num * 10 + (b - '0');}else{return minus ? -num : num;}b = rb();}}
            // Outputs
            public static void pln(){out.println();}
            public static void ps(String s){out.print(s);}
            public static void pi(int s){out.print(s);}
            public static void pl(long s){out.print(s);}
            public static void pd(double s){out.print(s);}
            public static void psp(){out.print(" ");}
            public static void parr(int s[]){out.println(Arrays.toString(s));}
            public static void parr(String s[]){out.println(Arrays.toString(s));}
            public static void parr(long s[]){out.println(Arrays.toString(s));}        
            public static void parr2(int s[][]){for(int i=0;i<s.length;i++)out.println(Arrays.toString(s[i]));}
            public static void parr2(long s[][]){for(int i=0;i<s.length;i++)out.println(Arrays.toString(s[i]));}
            public static void parr2(String s[][]){for(int i=0;i<s.length;i++)out.println(Arrays.toString(s[i]));}
            
            
    }   