import java.io.*;
import java.util.*;
import static java.lang.System.out;

public class Main {
    static MyReader in = new MyReader();

    public static void main(String[] args) {
        int N = in.i();
        int[] x = new int[N];
        int[] y = new int[N];
        for (int i = 0; i < N; i++) {
            x[i] = in.i();
            y[i] = in.i();
        }
        double[][] d = new double[N][N];
        for (int i = 0; i < N; i++) {
            for (int j = i + 1; j < N; j++) {
                double dx = x[i] - x[j];
                double dy = y[i] - y[j];
                d[i][j] = d[j][i] = Math.sqrt(dx * dx + dy * dy);
            }
        }
        int c = 1;
        for (int i = 2; i <= N; i++) {
            c *= i;
        }
        double ans = dfs(0, N, new boolean[N], new int[N], x, y, d) / c;
        out.println(ans);
    }

    static double dfs(int i, int n, boolean[] used, int[] arr, int[] x, int[] y, double[][] d) {
        if (i == n) {
            double l = 0;
            int s = arr[0];
            for (int j = 1; j < n; j++) {
                int t = arr[j];
                l += d[s][t];
                s = t;
            }
            return l;
        } else {
            double ans = 0;
            for (int j = 0; j < n; j++) {
                if (!used[j]) {
                    used[j] = true;
                    arr[i] = j;
                    ans += dfs(i + 1, n, used, arr, x, y, d);
                    used[j] = false;
                }
            }
            return ans;
        }
    }
}

class MyReader extends BufferedReader {
    char[] cbuf = new char[1024];
    int head = 0;
    int tail = 0;

    MyReader() {
        super(new InputStreamReader(System.in));
    }

    char next() {
        if (head == tail) {
            try {
                tail = super.read(cbuf, 0, cbuf.length);
            } catch (IOException e) {
                e.printStackTrace();
            }
            head = 0;
        }
        return cbuf[head++];
    }

    void back() {
        head--;
    }

    boolean minus() {
        boolean minus;
        while (true) {
            char c = next();
            if (!isDelimiter(c)) {
                if (!(minus = c == '-')) back();
                return minus;
            }
        }
    }

    void skip() {
        while (isDelimiter(next()));
        back();
    }

    char[] s(int N) {
        char[] cbuf = new char[N];
        read(cbuf, 0, N);
        return cbuf;
    }

    public int read(char[] cbuf, int off, int len) {
        skip();
        int i;
        for (i = 0; i < cbuf.length; i++) {
            char c = next();
            if (isDelimiter(c)) {
                break;
            }
            cbuf[i] = c;
        }
        return i;
    }

    boolean isDelimiter(char c) {
        return c == ' ' || c == '\n' || c == '\r';
    }

    int i() {
        boolean minus = minus();
        int n = 0;
        while (true) {
            int k = next() - '0';
            if (k < 0 || 9 < k) break;
            n = 10 * n + k;
        }
        return minus ? -n : n;
    }

    int[] ii(final int N) {
        int[] a = new int[N];
        for (int j = 0; j < a.length; j++) a[j] = i();
        return a;
    }

    long l() {
        boolean minus = minus();
        long n = 0;
        while (true) {
            int k = next() - '0';
            if (k < 0 || 9 < k) break;
            n = 10 * n + k;
        }
        return minus ? -n : n;
    }
}


Measured memory_bytes: 34364