#include <bits/stdc++.h>

using namespace std;

#define LL long long

#define INF 0x3f3f3f3f

#define mem(i,j) memset(i,j,sizeof(i))

#define inc(i,l,r) for(int i=l;i<=r;i++)

#define dec(i,r,l) for(int i=r;i>=l;i--)

#define gcd(i,j) __gcd(i,j);

const int N=2e5+;

const int mod=1e9+;

const double eps=1e-;

LL n;

map<LL,LL>mp1,mp2;

int main()

{

    while(~scanf("%lld",&n)) {

        mp1.clear(); mp2.clear();

        inc(i,,n) {

            LL x,y; scanf("%lld%lld",&x,&y);

            mp1[y-x]++; mp2[x+y]++;

        }

        double sum=n*n, ans=0.0;

        map<LL,LL>::iterator it;

        for(it=mp1.begin();it!=mp1.end();it++)

            ans+=it->second*(it->second-);

        for(it=mp2.begin();it!=mp2.end();it++)

            ans+=it->second*(it->second-);

        printf("%lf\n",ans/sum);

    }

    return ;

}