#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> ii;
typedef vector<int> vi;
typedef vector<ii> vii;
#define ALL(x) (x).begin(),(x).end()
#define ff first
#define ss second
#define D(a) cerr << ">> " << #a << " = >" << a << "<" << endl
#define PB push_back
#define F(i,a,b) for ( int i = (a); i < (b); ++i )
#define FD(i,a,b) for ( int i = (a); i >= (b); --i )
#define R1(a) scanf("%d",&a)
#define R2(a,b) scanf("%d%d",&a,&b)
#define R3(a,b,c) scanf("%d%d%d",&a,&b,&c)
#define DR1(a) int a; scanf("%d",&a)
#define DR2(a,b) int a,b; scanf("%d%d",&a,&b)
#define DR3(a,b,c) int a,b,c; scanf("%d%d%d",&a,&b,&c)

const int INF = 1e9+7;

const int DEBUG = 0;
const ii T = { 1, 100 };
const ii M = { 1, 100 };     // size of photo
const ii C = { -1e9, +1e9 }; // range of coordinates
const ii N = { 1, 1e6 };

bool between(int value, int lower, int upper) {
    return value >= lower && value <= upper;
}

int main( ) {
    int  n, x, y;
    char garbage;

    // assert(cin >> t && between(t, T.first, T.second));
    while (R1(n) == 1) {
        map<int,int> diag1, diag2;
        long         res = 0;
        assert(between(n, N.first, N.second));
        for (int i = 0; i < n; ++i) {
            assert(R2(x,y) == 2 && between(x, C.first, C.second) && between(y, C.first, C.second));
            int d1 = x + y;
            int d2 = x - y;
            res += diag1[d1];
            res += diag2[d2];
            ++diag1[d1];
            ++diag2[d2];
        }
        printf("%e\n", 2.0 * res / n / n);
    }
    assert(scanf("%c", &garbage) == EOF);
    return 0;
}