/*
#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
*/
#include <bits/stdc++.h>

#define rep(i,n)  for(int i=0; i<(n); i++)
using ll = long long;
using namespace std;

int main()
{
    int r;
    cin >> r;
    cout << (double)2.0 * r * 3.141592;
    return 0;
}
