import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }

string COLOR(string s = "") { return "\x1b[" ~ s ~ "m"; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


void bAdd(T)(T[] bit, int pos, T val)
in {
  assert(0 <= pos && pos < bit.length, "bAdd: 0 <= pos < |bit| must hold");
}
do {
  for (int x = pos; x < bit.length; x |= x + 1) {
    bit[x] += val;
  }
}

// sum of [0, pos)
T bSum(T)(T[] bit, int pos)
in {
  assert(0 <= pos && pos <= bit.length, "bSum: 0 <= pos <= |bit| must hold");
}
do {
  T ret = 0;
  for (int x = pos - 1; x >= 0; x = (x & (x + 1)) - 1) {
    ret += bit[x];
  }
  return ret;
}

T bSum(T)(T[] bit, int a, int b) {
  return bit.bSum(b) - bit.bSum(a);
}


alias Pt = Tuple!(int, "x", int, "y");

void main() {
  try {
    for (; ; ) {
      const N = readInt;
      auto P = new Pt[N];
      foreach (u; 0 .. N) {
        P[u].x = readInt;
        P[u].y = readInt;
      }
      P.sort;
      
      auto ys = new int[N];
      foreach (u; 0 .. N) ys[u] = P[u].y;
      ys = ys.sort.uniq.array;
      const ysLen = cast(int)(ys.length);
      
      auto bit = new int[ysLen];
      long[4] ans;
      ans[2] = ans[3] = 1L * N * (N-1) / 2;
      for (int i = 0, j; i < N; i = j) {
        for (; j < N && P[i].x == P[j].x; ++j) {}
        foreach (k; i .. j) {
          const pos = ys.lowerBound(P[k].y);
          ans[0] += bit.bSum(0, pos);
          ans[1] += bit.bSum(pos + 1, ysLen);
        }
        foreach (k; i .. j) {
          const pos = ys.lowerBound(P[k].y);
          bit.bAdd(pos, +1);
        }
        ans[2] -= 1L * (j-i) * (j-i-1) / 2;
      }
      foreach (i; 0 .. ysLen) {
        const n = bit.bSum(i, i + 1);
        ans[3] -= 1L * n * (n-1) / 2;
      }
      
      debug writeln(ans);
      writefln("%.20f", (ans[0] - ans[1]) / sqrt(1.0L * ans[2] * ans[3]));
    }
  } catch (EOFException e) {
  }
}