#include <bits/stdc++.h>

using namespace std;

const int N = 5e5+10;

double x[N], y[N];

inline double di(int i, int j) {
	return sqrt((x[i]-x[j])*(x[i]-x[j])+(y[i]-y[j])*(y[i]-y[j]));
}

int main() {
	int n; cin>>n;
	double ans=0;
	for(int i = 1; i <= n; i++)cin>>x[i]>>y[i];
	for(int i = 1; i <= n; i++) {
		for(int j = i+1; j <= n; j++) {
			ans+=di(i,j);
		}
	}
	cout<<ans;
	return 0;
}