#include <bits/stdc++.h>
using namespace std;

#define FOR(i,a,b) for(auto i=a;i<b;++i)
#define F(a) FOR(i,0l,a)
#define FF(a) FOR(j,0l,a)
#define aa first
#define bb second
#define PB push_back
#define pb push_back
#define MOD ((int)1e9+7)
#define INF (1<<30)
#define LINF (1L<<61)
#define EPS (1e-10)
#define EQ(a,b) (fabs(a-b)<=(fabs(a+b)*EPS))
#ifdef DEBUG
#define DEB puts("DEB!")
#define DPRINT(X) printf("#X: %d\n",X)
#define dout cout
#else
#define DEB do{}while(0)
#define DPRINT(X) do{}while(0)
#define dout 0 && cout
#endif
using ll=long long;
using ull=unsigned long long;
using ld=double;
using vi=vector<int>;
using vvi=vector<vi>;
using pii=pair<int,int>;

int main(){
  ios::sync_with_stdio(false);cout.tie(0);cin.tie(0);
  while (1) {
    map<ll,ll> d;
    map<ll,ll> d2;
    int n;
    cin >> n;
    if (!cin) break;
    for (int i = 0; i < n; ++ i) {
      int x, y;
      cin >> x >> y;
      int dif = x + y;
      d[dif] ++;
      d2[x - y] ++;
    }
    ll good = 0;
    for (auto i = d.begin(); i != d.end(); ++ i) {
      good += (((i->bb))*((i->bb)-1));
    }
    for (auto i = d2.begin(); i != d2.end(); ++ i) {
      good += (((i->bb))*((i->bb)-1));
    }
    double res = (double)(good) / (double)(n*n);
    cout << res << endl;
  }
  return 0;
}