#include <map>
#include <set>
#include <cmath>
#include <cstdio>
#include <vector>
#include <sstream>
#include <iostream>
#include <algorithm>
#define endl '\n'
#define all(var) var.begin(), var.end()
#define input freopen("input", "r", stdin)
#define output freopen("output", "w", stdout)
#define fast ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
int const MOD = 1e9+7;
typedef long long ll;
using namespace std;
 
int main(){
    double pi = acos(-1.0);
    double r; cin >> r;
    printf("%.20lf\n", 2*pi*r);
}
