#include <bits/stdc++.h>
using namespace std;
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
long long gcd(long long x, long long y) {
  long long tmp = 0;
  if (x < y) {
    tmp = x;
    x = y;
    y = tmp;
  }
  while (y > 0) {
    long long r = x % y;
    x = y;
    y = r;
  }
  return x;
}
long long lcm(long long x, long long y) { return x / gcd(x, y) * y; }
const int MAX = 1e6 + 1;
const long long MOD = 1e9 + 7;
long long fac[MAX], finv[MAX], inv[MAX];
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
long long COM(int n, int k) {
  if (n < k) return 0;
  if (n < 0 || k < 0) return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}
long long kaijo(long long k) {
  long long sum = 1;
  for (long long i = 1; i <= k; ++i) {
    sum *= i;
    sum %= 1000000000 + 7;
  }
  return sum;
}
long long modpow(long long a, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1) res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}
vector<bool> IsPrime;
void sieve(size_t max) {
  if (max + 1 > IsPrime.size()) {
    IsPrime.resize(max + 1, true);
  }
  IsPrime[0] = false;
  IsPrime[1] = false;
  for (size_t i = 2; i * i <= max; ++i)
    if (IsPrime[i])
      for (size_t j = 2; i * j <= max; ++j) IsPrime[i * j] = false;
}
struct UnionFind {
  vector<int> par;
  UnionFind(int n) : par(n, -1) {}
  int root(int x) {
    if (par[x] < 0)
      return x;
    else
      return par[x] = root(par[x]);
  }
  bool issame(int x, int y) { return root(x) == root(y); }
  bool merge(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y) return false;
    if (par[x] > par[y]) swap(x, y);
    par[x] += par[y];
    par[y] = x;
    return true;
  }
  int size(int x) { return -par[root(x)]; }
};
long long count(int n, int a) {
  long long bunshi = 1;
  for (int i = 0; i < a; i++) {
    bunshi *= (n - i);
    bunshi %= MOD;
  }
  long long bunbo = 1;
  for (int i = 1; i < a + 1; i++) {
    bunbo *= i;
    bunbo %= MOD;
  }
  bunbo = modpow(bunbo, MOD - 2, MOD);
  return (bunshi * bunbo) % MOD;
}
vector<long long> divisor(long long n) {
  vector<long long> ret;
  for (long long i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      ret.push_back(i);
      if (i * i != n) ret.push_back(n / i);
    }
  }
  sort(begin(ret), end(ret));
  return (ret);
}
const long long INF = 1e18;
int main() {
  double r;
  cin >> r;
  cout << fixed << setprecision(3) << 2.0 * r * 3.141592 << endl;
  return 0;
}
