#include <bits/stdc++.h>
using namespace std;

const int N = 5e5 + 5;
int n, x[N], y[N];
double ans;

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

int main() {
    scanf ("%d", &n);
    for (int i = 1; i <= n; ++i) scanf ("%d%d", x + i, y + i);
    for (int i = 1; i < n; ++i)
        for (int j = i + 1; j <= n; ++j) 
            ans += d(i, j);
    printf ("%lf\n", ans);
    return 0;
}