#include <bits/stdc++.h>
using namespace std;
const int lim = 1000000;
const int mod = 1e9;
int gcd(int a, int b) { return b ? gcd(b, a % b) : a; }
int lcm(int a, int b) { return a * b / gcd(a, b); }
inline int n_sum(int n) { return (n * (n + 1)) / 2; }
int power(int a, int b) {
  int res = 1;
  assert(b >= 0);
  for (; b; b >>= 1) {
    if (b & 1) res = res * a;
    a = a * a;
  }
  return res;
}
int powmod(int a, int b) {
  int res = 1;
  a %= mod;
  assert(b >= 0);
  for (; b; b >>= 1) {
    if (b & 1) res = res * a % mod;
    a = a * a % mod;
  }
  return res;
}
int gcdExtended(int a, int b, int &x, int &y) {
  if (a == 0) {
    x = 0;
    y = 1;
    return b;
  }
  int x1, y1, gcd = gcdExtended(b % a, a, x1, y1);
  x = y1 - ((b / a) * x1);
  y = x1;
  return gcd;
}
int modInv(int a, int m) {
  int x, y;
  gcdExtended(a, m, x, y);
  return (x % m + m) % m;
}
int divM(int a, int b) { return (modInv(b, mod) * (a % mod)) % mod; }
void _main(void) {
  int n;
  cin >> n;
  cout << (2 * 3.141 * n) << endl;
  return;
}
signed main(void) {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t = 1, i;
  for (i = 1; i < t + 1; ++i) {
    _main();
  }
  return 0;
}
