#include <bits/stdc++.h>
using namespace std;
int pr[50000 + 1], prnum[50000 + 1];
int sieve() {
  int i, j;
  for (i = 1; i <= 50000; i++) prnum[i] = i + 1;
  for (i = 1; i <= 50000; i++)
    if (prnum[i] != -1)
      for (j = 2 * prnum[i] - 1; j <= 50000; j += prnum[i]) prnum[j] = -1;
  j = 0;
  for (i = 1; i <= 50000; i++)
    if (prnum[i] != -1) pr[++j] = prnum[i];
  return j;
}
int f[10000] = {0};
long long fib(long long n) {
  if (n == 0) return 0;
  if (n == 1 || n == 2) return (f[n] = 1);
  if (f[n]) return f[n];
  long long k = (n & 1) ? (n + 1) / 2 : n / 2;
  f[n] = (n & 1) ? (fib(k) * fib(k) + fib(k - 1) * fib(k - 1))
                 : (2 * fib(k - 1) + fib(k)) * fib(k);
  return f[n];
}
bool prime(long long n) {
  if (n < 2) return false;
  for (long long i = 2; i * i <= n; i++) {
    if (n % i == 0) return false;
  }
  return true;
}
long long bino(long long n, long long k) {
  long long C[k + 1];
  memset(C, 0, sizeof(C));
  C[0] = 1;
  for (long long i = 1; i <= n; i++) {
    for (long long j = min(i, k); j > 0; j--) C[j] = C[j] + C[j - 1];
  }
  return C[k];
}
bool myComparison(const pair<int, int> &a, const pair<int, int> &b) {
  return a.second < b.second;
}
long long bigmod(long long a, long long p, long long m) {
  if (p == 0) return 1;
  if (p % 2) {
    return ((a % m) * (bigmod(a, p - 1, m))) % m;
  } else {
    long long c = bigmod(a, p / 2, m);
    return ((c % m) * (c % m)) % m;
  }
}
bool v(char a) {
  if (a == 'a' || a == 'e' || a == 'i' || a == 'o' || a == 'u') {
    return 1;
  } else
    return 0;
}
long long max(long long a, long long b) { return (a > b) ? a : b; }
long long min(long long a, long long b) { return (a > b) ? b : a; }
int lcs(string a, string b, int m, int n) {
  int L[m + 1][n + 1];
  for (long long i = 0; i < m + 1; i++) {
    for (long long j = 0; j < n + 1; j++) {
      if (i == 0 || j == 0)
        L[i][j] = 0;
      else if (a[i - 1] == b[j - 1])
        L[i][j] = L[i - 1][j - 1] + 1;
      else
        L[i][j] = max(L[i - 1][j], L[i][j - 1]);
    }
  }
  return L[m][n];
}
int bsc(int arr[], int n, int key) {
  int le = 0, ri = n;
  int mid;
  while (le < ri) {
    mid = (ri + le) >> 1;
    if (arr[mid] == key) {
      while (mid + 1 < n && arr[mid + 1] == key) mid++;
      break;
    } else if (arr[mid] > key)
      ri = mid;
    else
      le = mid + 1;
  }
  while (mid > -1 && arr[mid] > key) mid--;
  return mid + 1;
}
using namespace std;
int main() {
  double a;
  cin >> a;
  cout << setprecision(9) << 2 * a * acos(-1.0) << endl;
}
