import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.Deque;
import java.util.NoSuchElementException;


public class Main {
	public static void main(String[] args) {


		slover();
		out.flush();
	}
	static FastScanner sc = new FastScanner();
	static PrintWriter out = new PrintWriter(System.out);
	//StringBuffer S = new StringBuffer(sc.next());
		//String hoge2 = str.reverse().toString();
		//map.containsKey(A)
		//Map<Integer, Integer> map = new HashMap<Integer, Integer>(N);
		/*for ( キーのデータ型 key : マップの名前.keySet() ) {
			データのデータ型 data = マップの名前.get( key );
	 
			// keyやdataを使った処理;
		}*/
		//int i = Integer.parseInt(s);
		//Queue<String> qq=new ArrayDeque<>(); //add,poll,peek BFSは前から実行される
		//Deque<String> dd=new ArrayDeque<>();//push後ろに入れる,poll(pop)後ろからとる,peek addは先頭に入るからバグ注意
		//stackdfsは後ろから実行される
		//ArrayList<Integer> cc = new ArrayList<>(N);
		//cc.contains(tmp)
		//Arrays.asList(c).contains("a")
		//list.set(1,"walk");//1 1 2 3 5
	 
	 
		private static int INF =1000000007;
	 
		
		private static class D{
			
			int distance,i;String T;
			boolean s[];
			D(int i,int distance,boolean s[],String T ){
				this.distance=distance;
				this.i=i;
				this.s=s;
				this.T=T;
			}
			
			
		}
		
		
	private static int slover() {
		int N=sc.nextInt();
		int x[]=new int[N];
		int y[]=new int[N];
		
		Deque<D> dd=new ArrayDeque<>();//push後ろに入れる,poll(pop)後ろからとる,peek addは先頭に入るからバグ注意
		for(int i=0;i<N;i++) {
			x[i]=sc.nextInt();
			y[i]=sc.nextInt();
		}
		
		boolean s[]=new boolean[N];
		Arrays.fill(s, false);
		dd.add(new D(0,0,s,""));
		double ANS=0;
		int count=0;
		while(!dd.isEmpty()) {
			D cur =dd.poll();
			if(cur.i==N) {
				//p(cur.T);
				char v[]=cur.T.toCharArray();
				double ans=0;
				for(int i=0;i+1<cur.T.length();i++) {
					int t1=(int)v[i]-'0'-1;
					int t2=(int)v[i+1]-'0'-1;
					
					ans+=Math.pow((x[t1]-x[t2])*(x[t1]-x[t2])+(y[t1]-y[t2])*(y[t1]-y[t2]), 0.5);
					//p((double)(x[t1]-x[t2])*(x[t1]-x[t2])+(y[t1]-y[t2])*(y[t1]-y[t2]));
				}
				ANS+=ans;
				count++;
				//p(ans);
				
				
				
				
				continue;
			}
			for(int i=1;i<=N;i++) {
				if(cur.s[i-1])continue;
				cur.s[i-1]=true;
				dd.push(new D(cur.i+1,cur.distance,cur.s.clone(),cur.T+i));	
				cur.s[i-1]=false;
			}
			
		}
		
		p(ANS/count);
		
		
		
		
		return 0;
	}






	
	static void p(String ans) {out.println(ans);};
	static void p(int ans) {out.println(ans);};
	static void p() {out.println();};
	static void p(long ans) {out.println(ans);};
	static void p(double ans) {out.println(ans);};

	static void pn(String ans) {out.print(ans);};
	static void pn(int ans) {out.print(ans);};
	static void pn() {out.print("");};
	static void pn(long ans) {out.print(ans);};
	static void pn(double ans) {out.print(ans);};

	static class FastScanner {
	    private final InputStream in = System.in;
	    private final byte[] buffer = new byte[1024];
	    private int ptr = 0;
	    private int buflen = 0;

	    private boolean hasNextByte() {
	      if (ptr < buflen) {
	        return true;
	      } else {
	        ptr = 0;
	        try {
	          buflen = in.read(buffer);
	        } catch (IOException e) {
	          e.printStackTrace();
	        }
	        if (buflen <= 0) {
	          return false;
	        }
	      }
	      return true;
	    }

	    private int readByte() {
	      if (hasNextByte()) return buffer[ptr++];
	      else return -1;
	    }

	    private static boolean isPrintableChar(int c) {
	      return 33 <= c && c <= 126;
	    }

	    private void skipUnprintable() {
	      while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
	    }

	    public boolean hasNext() {
	      skipUnprintable();
	      return hasNextByte();
	    }

	    public String next() {
	      if (!hasNext()) throw new NoSuchElementException();
	      StringBuilder sb = new StringBuilder();
	      int b = readByte();
	      while (isPrintableChar(b)) {
	        sb.appendCodePoint(b);
	        b = readByte();
	      }
	      return sb.toString();
	    }

	    public long nextLong() {
	      if (!hasNext()) throw new NoSuchElementException();
	      long n = 0;
	      boolean minus = false;
	      int b = readByte();
	      if (b == '-') {
	        minus = true;
	        b = readByte();
	      }
	      if (b < '0' || '9' < b) {
	        throw new NumberFormatException();
	      }
	      while (true) {
	        if ('0' <= b && b <= '9') {
	          n *= 10;
	          n += b - '0';
	        } else if (b == -1 || !isPrintableChar(b)) {
	          return minus ? -n : n;
	        } else {
	          throw new NumberFormatException();
	        }
	        b = readByte();
	      }
	    }

	    public int nextInt() {
	      return (int) nextLong();
	    }

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

	    public int[] nextIntArray(int N, boolean oneBased) {
	      if (oneBased) {
	        int[] array = new int[N + 1];
	        for (int i = 1; i <= N; i++) {
	          array[i] = sc.nextInt();
	        }
	        return array;
	      } else {
	        int[] array = new int[N];
	        for (int i = 0; i < N; i++) {
	          array[i] = sc.nextInt();
	        }
	        return array;
	      }
	    }

	    public long[] nextLongArray(int N, boolean oneBased) {
	      if (oneBased) {
	        long[] array = new long[N + 1];
	        for (int i = 1; i <= N; i++) {
	          array[i] = sc.nextLong();
	        }
	        return array;
	      } else {
	        long[] array = new long[N];
	        for (int i = 0; i < N; i++) {
	          array[i] = sc.nextLong();
	        }
	        return array;
	      }
	    }
	  }
}



Measured memory_bytes: 39956