ll@N,@(X,Y)[N];
coordcomp(N,X);
coordcomp(N,Y);
vector<ll>Z[N];
fenwick<ll> fw;
ll P = 0,Q = 0;
rep(i,N)Z[X[i]].push_back(Y[i]);
fw.walloc(N,1);
rep(i,N){
for(int y:Z[i]){
P+=fw.range(0,y-1);
Q+=fw.range(y+1,N);
}
for(int y:Z[i]){
fw.add(y,1);
}
}
ll R=N*(N-1)/2,S=N*(N-1)/2;
ll cx[N]{},cy[N]{};
rep(i,N)cx[X[i]]++,cy[Y[i]]++;
rep(i,N)R-=cx[i]*(cx[i]-1)/2,S-=cy[i]*(cy[i]-1)/2;
double numer=P-Q;
double denom=std::sqrt(R)*std::sqrt(S);
wt(numer/denom);