import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.InputMismatchException;
import java.util.NoSuchElementException;
import java.util.StringJoiner;

class Main {

    static PrintWriter out;
    static InputReader sc;

    public static void main(String[] args) {
        sc = new InputReader(System.in);
        out = new PrintWriter(System.out);
        solve();
        out.flush();
    }

    static void solve() {
        int N = sc.nextInt();
        int[] x = new int[N];
        int[] y = new int[N];
        for (int i = 0; i < N; i++) {
            x[i] = sc.nextInt();
            y[i] = sc.nextInt();
        }
        int[] c = new int[N];
        for (int i = 0; i < N; i++) {
            c[i] = i;
        }
        Pair[] p = new Pair[N];
        double ans = 0;
        do {
            for (int i = 0; i < N; i++) {
                p[i] = new Pair(c[i], i);
            }
            Arrays.sort(p);
            double sum = 0;
            for (int i = 0; i < N - 1; i++) {
                sum += Math.sqrt(Math.pow(x[p[i].second] - x[p[i + 1].second], 2)
                        + Math.pow(y[p[i].second] - y[p[i + 1].second], 2));
            }
            ans += sum;
        } while ((c = nextPermutation(c)) != null);
        out.println(ans / kaijo(N));
    }

    static int kaijo(int n) {
        if (n == 1) {
            return 1;
        } else {
            return kaijo(n - 1) * n;
        }
    }

    /**
     * C++のPairと同じ
     */
    public static class Pair implements Comparable<Pair> {
        int first;
        int second;

        public Pair(int first, int second) {
            this.first = first;
            this.second = second;
        }

        @Override
        public int hashCode() {
            return Integer.valueOf(this.first + this.second).hashCode();
        }

        @Override
        public boolean equals(Object obj) {
            if (this == obj) {
                return true;
            }
            if (!(obj instanceof Pair)) {
                return false;
            }
            Pair pair = (Pair) obj;
            return this.first == pair.first && this.second == pair.second;
        }

        @Override
        public int compareTo(Pair o) {
            return Integer.compare(this.first, o.first);
        }

        @Override
        public String toString() {
            return String.format("{%d,%d}", this.first, this.second);
        }

    }

    // modifies c to next permutation or returns null if such permutation does not
    // exist
    public static int[] nextPermutation(final int[] c) {
        // 1. finds the largest k, that c[k] < c[k+1]
        int first = getFirst(c);
        if (first == -1) {
            return null; // no greater permutation
        }
        // 2. find last index toSwap, that c[k] < c[toSwap]
        int toSwap = c.length - 1;
        while (c[first] >= c[toSwap]) {
            --toSwap;
        }
        // 3. swap elements with indexes first and last
        swap(c, first++, toSwap);
        // 4. reverse sequence from k+1 to n (inclusive)
        toSwap = c.length - 1;
        while (first < toSwap) {
            swap(c, first++, toSwap--);
        }
        return c;
    }

    // finds the largest k, that c[k] < c[k+1]
    // if no such k exists (there is not greater permutation), return -1
    private static int getFirst(final int[] c) {
        for (int i = c.length - 2; i >= 0; --i) {
            if (c[i] < c[i + 1]) {
                return i;
            }
        }
        return -1;
    }

    // swaps two elements (with indexes i and j) in array
    private static void swap(final int[] c, final int i, final int j) {
        final int tmp = c[i];
        c[i] = c[j];
        c[j] = tmp;
    }

    static void debug(Object... args) {
        StringJoiner j = new StringJoiner(" ");
        for (Object arg : args) {
            Class<?> type = arg.getClass();
            if (!type.isArray()) {
                j.add(arg.toString());
            } else if (type.getComponentType().isArray()) {
                for (Object o : (Object[]) arg) {
                    debug(o);
                }
            } else if (arg instanceof int[]) {
                j.add(Arrays.toString((int[]) arg));
            } else if (arg instanceof long[]) {
                j.add(Arrays.toString((long[]) arg));
            } else if (arg instanceof double[]) {
                j.add(Arrays.toString((double[]) arg));
            } else if (arg instanceof char[]) {
                j.add(Arrays.toString((char[]) arg));
            } else if (arg instanceof boolean[]) {
                j.add(Arrays.toString((boolean[]) arg));
            } else if (arg instanceof Object[]) {
                j.add(Arrays.toString((Object[]) arg));
            } else {
                j.add(arg.toString());
            }
        }
        System.err.println(j.toString());
    }

    static class InputReader {

        private InputStream in;
        private byte[] buffer = new byte[1024];
        private int curbuf;
        private int lenbuf;

        public InputReader(InputStream in) {
            this.in = in;
            this.curbuf = this.lenbuf = 0;
        }

        public boolean hasNextByte() {
            if (this.curbuf >= this.lenbuf) {
                this.curbuf = 0;
                try {
                    this.lenbuf = this.in.read(this.buffer);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (this.lenbuf <= 0) {
                    return false;
                }
            }
            return true;
        }

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

        private boolean isSpaceChar(int c) {
            return !(c >= 33 && c <= 126);
        }

        private void skip() {
            while (this.hasNextByte() && this.isSpaceChar(this.buffer[this.curbuf])) {
                this.curbuf++;
            }
        }

        public boolean hasNext() {
            this.skip();
            return this.hasNextByte();
        }

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

        public int nextInt() {
            if (!this.hasNext()) {
                throw new NoSuchElementException();
            }
            int c = this.readByte();
            while (this.isSpaceChar(c)) {
                c = this.readByte();
            }
            boolean minus = false;
            if (c == '-') {
                minus = true;
                c = this.readByte();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res = res * 10 + c - '0';
                c = this.readByte();
            } while (!this.isSpaceChar(c));
            return minus ? -res : res;
        }

        public long nextLong() {
            if (!this.hasNext()) {
                throw new NoSuchElementException();
            }
            int c = this.readByte();
            while (this.isSpaceChar(c)) {
                c = this.readByte();
            }
            boolean minus = false;
            if (c == '-') {
                minus = true;
                c = this.readByte();
            }
            long res = 0;
            do {
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res = res * 10 + c - '0';
                c = this.readByte();
            } while (!this.isSpaceChar(c));
            return minus ? -res : res;
        }

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

        public int[] nextIntArray(int n) {
            int[] a = new int[n];
            for (int i = 0; i < n; i++) {
                a[i] = this.nextInt();
            }
            return a;
        }

        public long[] nextLongArray(int n) {
            long[] a = new long[n];
            for (int i = 0; i < n; i++) {
                a[i] = this.nextLong();
            }
            return a;
        }

        public char[][] nextCharMap(int n, int m) {
            char[][] map = new char[n][m];
            for (int i = 0; i < n; i++) {
                map[i] = this.next().toCharArray();
            }
            return map;
        }
    }
}

Measured memory_bytes: 32024