#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define int long long

#define LOG(variable) cout << #variable":\t" << (variable) << endl
#define LOGCON(i, container) for(int (i) = 0; (i) < (container).size(); ++(i)) cout << (i) << ":\t" << (container)[(i)] << endl
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define REPS(i, r, n) for (int i = (r); i < (n); ++i)
#define REPR(i, n) for(int i = (n); i >= 0; --i) // from n to 0
#define REPRS(i, n, r) for(int i = (n); i >= (r); --i) // from n to r
#define REPOBJ(itr, obj) for(auto itr = (obj).begin(); itr != (obj).end() ; ++itr)
#define REPROBJ(itr, obj) for(auto itr = (obj).rbegin(), e = (obj).rend(); itr != e; ++itr)
#define COUTB(x) cout << (x) << "\n"
#define COUTS(x) cout << (x) << " "
#define PB push_back
#define SORT(obj) sort((obj).begin(), (obj).end())
#define SORTR(obj) sort((obj).begin(), (obj).end(), greater<>())
#define ALL(obj) (obj).begin(), (obj).end()
#define MOD 1000000007
#define PI (acos(-1))
struct mint {
  ll x; // typedef long long ll;
  mint(ll x=0):x((x%MOD+MOD)%MOD){}
  mint operator-() const { return mint(-x);}
  mint& operator+=(const mint a) {
    if ((x += a.x) >= MOD) x -= MOD;
    return *this;
  }
  mint& operator-=(const mint a) {
    if ((x += MOD-a.x) >= MOD) x -= MOD;
    return *this;
  }
  mint& operator*=(const mint a) { (x *= a.x) %= MOD; return *this;}
  mint operator+(const mint a) const { return mint(*this) += a;}
  mint operator-(const mint a) const { return mint(*this) -= a;}
  mint operator*(const mint a) const { return mint(*this) *= a;}
  mint pow(ll t) const {
    if (!t) return 1;
    mint a = pow(t>>1);
    a *= a;
    if (t&1) a *= *this;
    return a;
  }

  // for prime MOD
  mint inv() const { return pow(MOD-2);}
  mint& operator/=(const mint a) { return *this *= a.inv();}
  mint operator/(const mint a) const { return mint(*this) /= a;}
};
istream& operator>>(istream& is, const mint& a) { return is >> a.x;}
ostream& operator<<(ostream& os, const mint& a) { return os << a.x;}
int in() {int x; cin>>x; return x;}
string stin() {string s; cin>>s; return s;}

int lcm(int, int);
map<int,int> GetPrimeFactors(int);
bool IsPrime(int);
int Factorial(int);
int npr (int, int);
int ncr (int, int);

/***** MAIN *****/
void Main() {
  int n = in();
  cout << 2 * n * PI;
  cout << "\n";
}
/***** MAIN *****/

signed main() {
  cin.tie(0);
  ios_base::sync_with_stdio(false);
  cout << fixed << setprecision(15);
  Main();
  return 0;
}

int lcm(int a, int b) { return a / __gcd(a,b) * b;}

map<int,int> GetPrimeFactors(int n) {
  map<int,int> mp;
  // divide by 2, 3, 5, ...
  while(n%2 == 0) {
    ++mp[2];
    n /= 2;
  }
  for(int i=3; i*i<=n; i+=2) {
    while(n%i == 0) {
      ++mp[i];
      n /= i;
    }
  }
  if(n!=1) ++mp[n];

  return mp;
}

bool IsPrime(int a) {
  if(a <= 1) return false;
  if(a == 2) return true;
  if(a%2 == 0) return false;
  for(int i = 3; i*i <= a; i+=2) {
    if(a%i==0) return false;
  }
  return true;
}

int Factorial(int n){
  int ans = 1;
  while(n > 1){
    ans *= n;
    --n;
  }
  return ans;
}

int npr(int n, int r) {
  return Factorial(n)/Factorial(n-r);
}

int ncr (int n, int r){
  return npr(n,r)/Factorial(r);
}


Measured memory_bytes: 3812