#include <bits/stdc++.h>
#include <cstdlib>

using namespace std;

int main(int argc, char** argv) {
    ios::sync_with_stdio(false);
    
    int N;
    while(cin >> N) {
        
        vector< pair<int, int> > points(N);
        for(int i = 0; i <N; ++i) {
            cin >> points[i].first >> points[i].second;
        }
        double x = 1, y = 1;
        vector<int> nw, sw;
        for(int i = 0; i < N; ++i) {
            nw.push_back(abs(x*points[i].first + y*points[i].second));
            if (points[i].first * x + points[i].second * y > 0) {
                nw[i] *= -1;
            }
        }
        y = -1;
        for(int i = 0; i < N; ++i) {
            sw.push_back(abs(x*points[i].first + y*points[i].second));
            if (points[i].first * x + points[i].second * y > 0) {
                sw[i] *= -1;
            }
        }
        sort(nw.begin(), nw.end());
        sort(sw.begin(), sw.end());
//        for(int i = 0; i < N; ++i) {
//            cout << nw[i] << " ";
//        } cout << endl;
//                for(int i = 0; i < N; ++i) {
//            cout << sw[i] << " ";
//        } cout << endl;
        
        double d = nw[0];
        vector<int> nwcounts, swcounts;
        int count = 1;
        for(int i = 1; i < N; ++i) {
            if(nw[i] == d) {
                ++count;
            } else {
                nwcounts.push_back(count); count = 1;
                d = nw[i];
            }
        }
        nwcounts.push_back(count);
        count = 1; d = sw[0];
        for(int i = 1; i < N; ++i) {
            if(sw[i] == d) {
                ++count;
            } else {
                swcounts.push_back(count); count = 1;
                d = sw[i];
            }
        }
        swcounts.push_back(count);
        
//        for(size_t i = 0 ; i < nwcounts.size(); ++i) {
//            cout << nwcounts[i] << " ";
//        } cout << endl;
//                
//        for(size_t i = 0; i < swcounts.size(); ++i) {
//            cout << swcounts[i] << " ";
//        } cout << endl;
        
//        double prob = 0; double S; double P = N;
//        for(int i = 0; i < nwcounts.size(); ++i) {
//            S = nwcounts[i];
//            prob+= ((S*(S-1))/2)/(P*(P-1));
//        }
//        for(int i = 0; i < swcounts.size(); ++i) {
//            S = swcounts[i];
//            prob += ((S*(S-1))/2)/(P*(P-1));
//        }
        double prob = 0;
        for(size_t i = 0; i < nwcounts.size(); ++i) {
            prob += (nwcounts[i])*(nwcounts[i]-1);
            
        }
        for(size_t i = 0; i < swcounts.size(); ++i) {
            prob += swcounts[i] * (swcounts[i]-1);
        }
        prob = double(prob) / (double(N)*N);
        cout << prob << endl;
    }
    return 0;
}