#include <bits/stdc++.h>
using namespace std;
long long mod = 1e9;
long long MOD = 1e18;
bool comp(const pair<int, int>& p, const pair<int, int>& q) {
  return (p.first < q.first);
}
bool comp2(const pair<int, int>& p, const pair<int, int>& q) {
  if (p.second != q.second) return (p.second < q.second);
  return (p.first < q.first);
}
bool comp3(const pair<long long, pair<long long, long long> >& p,
           const pair<long long, pair<long long, long long> >& q) {
  if (p.first != q.first) return p.first < q.first;
  if (p.second.first != q.second.first) return p.second.first < q.second.first;
  return p.second.second < q.second.second;
}
long long pp(long long a, long long b) {
  long long t = 1;
  while (b) {
    if (b & 1) t = (t * a) % mod;
    a = (a * a) % mod;
    b /= 2;
  }
  return t;
}
bool isprime(int n) {
  int i = 2;
  if (n == 1) return true;
  if (n == 2) return true;
  for (; i * i <= n; i++) {
    if (n % i == 0) return false;
  }
  return true;
}
long long lcd(long long n, long long m) {
  if (n > m) swap(m, n);
  if (m % n == 0)
    return m;
  else {
    int p = n + n;
    while (1) {
      if (p % m == 0)
        return p;
      else
        p = (n + p) % MOD;
    }
  }
}
void print(long long n) {
  long long mask = 1LL << 63;
  for (long long i = 0; i < 64; i++) {
    int p = (mask & n) ? 1 : 0;
    n = n << 1;
    cout << p;
  }
  cout << endl;
}
int getint() {
  int res;
  scanf("%d", &res);
  return res;
}
void printint(int n) { printf("%d\n", n); }
int v[100];
int N;
int cnt_cycle(int x) {
  int a = v[x];
  int b = v[v[x]];
  cout << a << ' ' << b << endl;
  cout << "1st loop" << endl;
  while (a != b) {
    a = v[a];
    b = v[v[b]];
    cout << a << ' ' << b << endl;
  }
  a = x;
  cout << "2nd loop " << endl;
  while (a != b) {
    a = v[a];
    b = v[b];
    cout << a << ' ' << b << endl;
  }
  b = v[a];
  int c = 1;
  cout << b << " 3rd loop " << endl;
  while (a != b) {
    b = v[b];
    cout << a << ' ' << b << endl;
    c++;
  }
  return c;
}
int main() {
  double r;
  cin >> r;
  cout << 2 * r * acos(-1);
}
