#define ATCODER #define _USE_MATH_DEFINES #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include using namespace std; using ll = long long; using ld = long double; using pll = pair; using pdd = pair; //template using pq = priority_queue, greater>; #define FOR(i, a, b) for(ll i=(a); i<(b);i++) #define REP(i, n) for(ll i=0; i<(n);i++) #define ROF(i, a, b) for(ll i=(b-1); i>=(a);i--) #define PER(i, n) for(ll i=n-1; i>=0;i--) #define REPREP(i,j,a,b) for(ll i=0;i > #define VV2(type,n,m,val) vector< vector > val;val.resize(n);for(ll i;i #define VEC(type,n,val) vector val;val.resize(n) #define VL vector #define VVL vector< vector > #define VP vector< pair > #define SZ size() #define all(i) begin(i),end(i) #define SORT(i) sort(all(i)) #define BITI(i) (1<>i) & 1) != 0) #define ALLBIT(n) (ll(1)<v?v:n #define MP(a,b) make_pair(a,b) #define DET2(x1,y1,x2,y2) (x1)*(y2)-(x2)*(y1) #define DET3(x1,y1,z1,x2,y2,z2,x3,y3,z3) (x1)*(y2)*(z3)+(x2)*(y3)*(z1)+(x3)*(y1)*(z2)-(z1)*(y2)*(x3)-(z2)*(y3)*(x1)-(z3)*(y1)*(x2) #define INC(a) for(auto& v:a)v++; #define DEC(a) for(auto& v:a)v--; #define SQU(x) (x)*(x) #define L0 ll(0) #ifdef ATCODER #include using namespace atcoder; using mint = modint1000000007; using mint2 = modint998244353; #endif template vector read(size_t n) { vector ts(n); for (size_t i = 0; i < n; i++) cin >> ts[i]; return ts; } template void read_tuple_impl(TV&) {} template void read_tuple_impl(TV& ts) { get(ts).emplace_back(*(istream_iterator(cin))); read_tuple_impl(ts); } template decltype(auto) read_tuple(size_t n) { tuple...> ts; for (size_t i = 0; i < n; i++) read_tuple_impl(ts); return ts; } using val = ll; using func = pair; val opm(val a, val b) { return min(a, b); } val em() { return 1e18; } val opM(val a, val b) { return max(a, b); } val eM() { return 0; } //val mp(func f, val a) //{ // if (f.first < 0) // return a; // return f; //} //func comp(func f, func g) { // if (g.first < 0) // return f; // return g; //} // //func id() { // return MP(-1, -1); //} ll di[4] = { 1,0,-1,0 }; ll dj[4] = { 0,1,0,-1 }; ll si[4] = { 0,3,3,0 }; ll sj[4] = { 0,0,3,3 }; //ll di[4] = { -1,-1,1,1 }; //ll dj[4] = { -1,1,-1,1 }; ll di8[8] = { 0,-1,-1,-1,0,1,1,1 }; ll dj8[8] = { -1,-1,0,1,1,1,0,-1 }; class BIT { public: BIT(ll s) { size = 1; while (size < s) size *= 2; bit.assign(size, 0); // def unit } void update(ll k, ll var) { k += 1; while (k <= size) { bit[k - 1] += var; // def func k += k & -k; } } ll query(ll a) { ll s = 0; a++; while (a > 0) { s += bit[a - 1]; // def func a -= a & -a; } return s; } ll size; vector bit; }; void solve() { ll n; cin >> n; auto [x, y] = read_tuple(n); unordered_map xc, yc; VP xy(n); REP(i, n) { xc[x[i]]++; yc[y[i]]++; xy[i] = MP(x[i], y[i]); } SORT(xy); y.push_back(-2000000000); y.push_back(2000000000); SORT(y); y.erase(unique(all(y)), y.end()); unordered_map ry; REP(i, y.size()) { ry[y[i]] = i; } BIT bit(y.size()); ll p = 0, q = 0, r = 0, s = 0; for (auto& [k, c] : xc) { r += c * (c - 1) / 2; } for (auto& [k, c] : yc) { s += c * (c - 1) / 2; } r = n * (n - 1) / 2 - r; s = n * (n - 1) / 2 - s; ll add = 0; REP(i, n) { if (i > 0 && xy[i - 1].first != xy[i].first) { FOR(j, add, i) { bit.update(ry[xy[j].second], 1); } add = i; } p += bit.query(ry[xy[i].second] - 1); q += bit.query(y.size()) - bit.query(ry[xy[i].second]); } printf("%.15f", (p - q) / sqrt(r * s)); return; } int main() { ll t = 1; //cin >> t; while (t--) { solve(); } return 0; }