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) { } }