#include <bits/stdc++.h>
using namespace std;
int get(int n, int p) {
  int res = 0;
  while (n >= p) res += n / p, n /= p;
  return res;
}
template <class T>
T power(T N, T P) {
  return (P == 0) ? 1 : N * power(N, P - 1);
}
struct hash_pair {
  template <class T1, class T2>
  size_t operator()(const pair<T1, T2>& p) const {
    auto hash1 = hash<T1>{}(p.first);
    auto hash2 = hash<T2>{}(p.second);
    return hash1 ^ hash2;
  }
};
template <typename T>
void print(const T& v) {
  cerr << v << ' ';
}
template <typename T1, typename... T2>
void print(const T1& first, const T2&... rest) {
  print(first);
  print(rest...);
}
const int Maxn = 1e5 + 1;
template <typename T, typename T1>
T mod(T x, T1 p) {
  x %= p;
  if (x < 0) x += p;
  return x;
}
int fact[Maxn];
void factorial() {
  fact[0] = 1;
  for (int i = 1; i < Maxn; i++) {
    fact[i] = 1LL * fact[i - 1] * i % 1000000007;
  }
}
int inv[Maxn], ifact[Maxn];
template <typename T>
T inverse(T x, T p) {
  x = mod(x, p);
  if (x == 1) return x;
  return mod((1LL * (-p / x) * (inv[p % x] % p)), p);
}
void inverse_fact() {
  ifact[0] = 1;
  for (int i = 1; i < Maxn; i++) {
    inv[i] = inverse(i, 1000000007);
    ifact[i] = (1LL * ifact[i - 1] * inv[i]) % 1000000007;
  }
}
int nCr(int n, int r) {
  int ret = (1LL * ifact[n - r] * ifact[r]) % 1000000007;
  ret = (1LL * ret * fact[n]) % 1000000007;
  return ret;
}
bool cmp(int a, int b) { return a > b; }
int main() {
  int n;
  cin >> n;
  printf("%0.5f\n", n * 2.0 * 2 * acos(0.0));
  return 0;
}
