/*
* Note: This template uses some c++11 functions , so you have to compile it with c++11 flag.
*       Example:-   $ g++ -std=c++11 c++Template.cpp
*
* Author : Akshay Pratap Singh
* Handle: code_crack_01
*
*/
 
/********   All Required Header Files ********/
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <sstream>
#include <queue>
#include <deque>
#include <bitset>
#include <iterator>
#include <list>
#include <stack>
#include <map>
#include <set>
#include <functional>
#include <numeric>
#include <utility>
#include <limits>
#include <time.h>
#include <math.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>

using namespace std;

/*******  All Required define Pre-Processors and typedef Constants *******/
#define SCD(t) scanf("%d",&t)
#define SCLD(t) scanf("%ld",&t)
#define SCLLD(t) scanf("%lld",&t)
#define SCC(t) scanf("%c",&t)
#define SCS(t) scanf("%s",t)
#define SCF(t) scanf("%f",&t)
#define SCLF(t) scanf("%lf",&t)
#define MEM(a, b) memset(a, (b), sizeof(a))
#define FOR(i, j, k, in) for (int i=j ; i<k ; i+=in)
#define RFOR(i, j, k, in) for (int i=j ; i>=k ; i-=in)
#define REP(i, j) FOR(i, 0, j, 1)
#define RREP(i, j) RFOR(i, j, 0, 1)
#define all(cont) cont.begin(), cont.end()
#define rall(cont) cont.end(), cont.begin()
#define FOREACH(it, l) for (auto it = l.begin(); it != l.end(); it++)
#define IN(A, B, C) assert( B <= A && A <= C)
#define MP make_pair
#define PB push_back
#define INF (int)1e9
#define EPS 1e-9
#define PI 3.1415926535897932384626433832795
#define MOD 1000000007
#define read(type) readInt<type>()
const double pi=acos(-1.0);
typedef pair<int, int> PII;
typedef vector<int> VI;
typedef vector<string> VS;
typedef vector<PII> VII;
typedef vector<VI> VVI;
typedef map<int,int> MPII;
typedef set<int> SETI;
typedef multiset<int> MSETI;
typedef long int int32;
typedef unsigned long int uint32;
typedef long long int int64;
typedef unsigned long long int  uint64;

/****** Template of some basic operations *****/
template<typename T, typename U> inline void amin(T &x, U y) { if(y < x) x = y; }
template<typename T, typename U> inline void amax(T &x, U y) { if(x < y) x = y; }
/**********************************************/

/****** Template of Fast I/O Methods *********/
template <typename T> inline void write(T x)
{
	int i = 20;
	char buf[21];
	// buf[10] = 0;
	buf[20] = '\n';

	do
	{
		buf[--i] = x % 10 + '0';
		x/= 10;
	}while(x);
	do
	{
		putchar(buf[i]);
	} while (buf[i++] != '\n');
}

template <typename T> inline T readInt()
{
	T n=0,s=1;
	char p=getchar();
	if(p=='-')
		s=-1;
	while((p<'0'||p>'9')&&p!=EOF&&p!='-')
		p=getchar();
	if(p=='-')
		s=-1,p=getchar();
	while(p>='0'&&p<='9') {
		n = (n<< 3) + (n<< 1) + (p - '0');
		p=getchar();
	}

	return n*s;
}
/************************************/


/******* Debugging Class Template *******/
#define DEBUG

#ifdef DEBUG

    #define debug(args...)     (Debugger()) , args

    class Debugger
    {
        public:
        Debugger(const std::string& _separator = " - ") :
        first(true), separator(_separator){}

        template<typename ObjectType> Debugger& operator , (const ObjectType& v)
        {
            if(!first)
                std:cerr << separator;
            std::cerr << v;
            first = false;
            return *this;
        }
        ~Debugger() {  std:cerr << endl;}

        private:
        bool first;
        std::string separator;
    };

#else
    #define debug(args...)                  // Just strip off all debug tokens
#endif

/**************************************/

/******** User-defined Function *******/


/**************************************/


/********** Main()  function **********/
int main()
{

	double r;
  cin >> r;
  cout << 2 * PI * r << endl;
	return 0;
}
/********  Main() Ends Here *************/

Measured memory_bytes: 3752