//omajinai c++ atcoder
#include <iostream> 
#include <string> 
#include <vector> 
#include <algorithm> 
#include <utility> 
#include <tuple> 
#include <cstdint> 
#include <cstdio> 
#include <map> 
#include <queue> 
#include <set> 
#include <stack> 
#include <deque> 
#include <unordered_map> 
#include <unordered_set> 
#include <bitset> 
#include <cctype> 
#include<cmath>
#include<iomanip>
#include<type_traits>
//#include<numbers>
#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define all(x) (x).begin(),(x).end()
#define itn int
#define yes() printf("Yes\n")
#define no() printf("No\n")
typedef long long ll;
constexpr auto mod = 1000000007;
using namespace std;

int main() {


    long double pi=3.141592653589793;
    double r;
    cin>>r;
    cout<<2*pi*r;
}