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

#define PB push_back
#define CL(A, I) (memset(A, I, sizeof(A)))
#define D(X) cout<<"  "<<#X": "<<X<<endl;
#define FOR(i, m, n) for(int i(m); i < n; i++)
#define F(n) FOR(i, 0, n)
#define FF(n) FOR(j, 0, n)
#define aa first
#define bb second

typedef long long ll;
typedef vector<ll> vi;
typedef pair<ll, ll> ii;
typedef vector<ii> vii;

int n;
map<ll, ll> ne, nw;

int main() {
    while (scanf("%d", &n) == 1) {
        ne.clear();
        nw.clear();
        F(n) {
            ll x, y; scanf("%lld%lld", &x, &y);
            ne[x-y]++;
            nw[x+y]++;
        }
        double tot = n*n;
        double prs = 0;
        for (auto i : ne) prs += i.bb * (i.bb-1);
        for (auto i : nw) prs += i.bb * (i.bb-1);
        printf("%.10lf\n", prs/tot);
    }
}