#include <bits/stdc++.h>
#define REP(A,B) for(int (A)=0;(A)<(B);(A)++)
using namespace std;
int XI[1111111], YI[1111111];
int R[1111111];
int C[1111111];
int main()
{
	int n;
	while(scanf("%d", &n) == 1) {
		map<int, int > M,M2;
		set<int> indx, indx2;
		REP(i, n) {
			scanf("%d%d", XI+i, YI+i);
			M[XI[i]-YI[i]]++;
			indx.insert(XI[i]-YI[i]);
			indx2.insert(XI[i]+YI[i]);
			M2[XI[i]+YI[i]]++;
		}
		long double ans = 0.0;
		for(int i1 : indx) {
			long long c = M[i1];
//			printf("i1=%d, c=%lld\n", i1, c);
			ans += c*(c-1)/2;
		}
		for(int i1 : indx2) {
			long long c = M2[i1];
//			printf("i2=%d, c=%lld\n", i1, c);
			ans += c*(c-1)/2;
		}
		ans /= ((long long)n)*n/2.0;
		printf("%0.6lf\n", (double)ans);
	}
	return 0;
}