#include <bits/stdc++.h>
using namespace std;
const int mod = 1000000007;
void add(long long int& a, long long int b) {
  a += b;
  while (a >= mod) a -= mod;
  while (a < 0) a += mod;
}
void mul(long long int& a, long long int b) {
  a *= b;
  while (a >= mod) a -= mod;
  while (a < 0) a += mod;
}
void up_self(long long int& a, long long int b) { a = max(a, b); }
void min_self(long long int& a, long long int b) { a = min(a, b); }
long long int binexpomodulo(long long int x, long long int y) {
  long long int res = 1;
  x %= mod;
  if (!x) return 0;
  while (y) {
    if (y & 1) {
      mul(res, x);
    }
    mul(x, x);
    y >>= 1;
  }
  return res;
}
long long int nCrInOr(long long int n, long long int r) {
  long long int res = 1;
  if (r > n - r) r = n - r;
  long long int rin = 1;
  for (long long int i = 1; i <= r; i++) rin = (rin * i) % mod;
  rin = binexpomodulo(rin, mod - 2);
  for (long long int i = 1; i <= r; i++) res = (res * (n - i + 1)) % mod;
  res = (res * rin) % mod;
  return res;
}
int msb(long long int n) {
  int ans;
  for (int i = 0; i < 64; i++)
    if (n & (1LL << i)) ans = i + 1;
  return ans;
}
void strrev(string& second) {
  int n = (second.size());
  for (int i = 0; i < n / 2; i++) swap(second[i], second[n - 1 - i]);
}
int main() {
  double r;
  cin >> r;
  cout << fixed << setprecision(5) << 2 * 3.14159265359 * r << endl;
  return 0;
}
