import java.util.*;
import java.io.*;

public class Main {
    public static void main(String[] args) {
        FastScanner sc = new FastScanner();
        int n = sc.nextInt();
        List<Pair> p = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            int x = sc.nextInt();
            int y = sc.nextInt();
            p.add(new Pair(x, y));
        }
        int[] data = new int[n];
        for (int i = 0; i < n; i++) {
            data[i] = i;
        }

        double ans = 0;
        double cnt = 0;
        do {
            cnt++;
            double dist = 0;
            for (int i = 0; i < n - 1; i++) {
                double x = Math.abs(p.get(data[i]).x - p.get(data[i + 1]).x);
                double y = Math.abs(p.get(data[i]).y - p.get(data[i + 1]).y);
                dist += Math.sqrt(x * x + y * y);
            }
            ans += dist;
        } while (findNextPermutation(data));

        ans /= cnt;
        System.out.printf("%.9f%n", ans);

    }

    public static boolean findNextPermutation(int data[]) {

        // If the given dataset is empty
        // or contains only one element
        // next_permutation is not possible
        if (data.length <= 1)
            return false;

        int last = data.length - 2;

        // find the longest non-increasing suffix
        // and find the pivot
        while (last >= 0) {
            if (data[last] < data[last + 1]) {
                break;
            }
            last--;
        }
        if (last < 0)
            return false;

        int nextGreater = data.length - 1;

        // Find the rightmost successor to the pivot
        for (int i = data.length - 1; i > last; i--) {
            if (data[i] > data[last]) {
                nextGreater = i;
                break;
            }
        }

        // Swap the successor and the pivot
        data = swap(data, nextGreater, last);

        // Reverse the suffix
        data = reverse(data, last + 1, data.length - 1);
        return true;
    }

    public static int[] swap(int data[], int left, int right) {
        // Swap the data
        int temp = data[left];
        data[left] = data[right];
        data[right] = temp;
        // Return the updated array
        return data;
    }

    public static int[] reverse(int data[], int left, int right) {
        // Reverse the sub-array
        while (left < right) {
            int temp = data[left];
            data[left++] = data[right];
            data[right--] = temp;
        }
        // Return the updated array
        return data;
    }
}

class Pair {
    long x;
    long y;

    public Pair(long x, long y) {
        this.x = x;
        this.y = y;
    }

    public long compareTo(Pair p) {
        return this.y - p.y;
    }

    public boolean equals(Object o) {
        if (o == this) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }
        Pair p = (Pair) o;
        return this.x == p.x && this.y == p.y;
    }

    public int hashCode() {
        return Objects.hash(x, y);
    }

    public String toString() {
        return String.format("x = %s,y = %s", x, y);
    }
}

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;
    }

    public boolean hasNext() {
        while (hasNextByte() && !isPrintableChar(buffer[ptr]))
            ptr++;
        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() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE)
            throw new NumberFormatException();
        return (int) nl;
    }

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


Measured memory_bytes: 23016