#include <algorithm>
#include <bitset>
#include <boost/multiprecision/cpp_int.hpp>
#include <algorithm>
#include <climits>
#include <cmath>
#include <cstring>
#include <functional>
#include <iostream>
#include <list>
#include <map>
#include <set>
#include <numeric>
#include <sstream>
#include <stdio.h>
#include <string>
#include <tuple>
#include <vector>
#include<iomanip>
#include <cmath>
#define _USE_MATH_DEFINES
namespace mp = boost::multiprecision;
using namespace std;
using ull = unsigned long long;
using ld = long double;
using lll = boost::multiprecision::cpp_int;
using ll = long long;
#define FOR(i, N) for(ll i = 0; i < N; ++i)

#define ALL(v) (v).begin(), (v).end()
#define RALL(v) (v).rbegin(), (v).rend()
int main(){
  ll R;
  cin>>R;
  printf("%.5lf\n", R*2*M_PI);
  return 0;
}