#include <bits/stdc++.h>
using namespace std;
long long power(long long a, long long b) {
  long long res = 1;
  for (long long i = 0; i < b; ++i) {
    res *= a;
  }
  return res;
}
long long logx(long long base, long long num) {
  int cnt = 0;
  while (num != 1) {
    num /= base;
    ++cnt;
  }
  return cnt;
}
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long lcm(long long a, long long b) { return (a * b) / gcd(a, b); }
void print(vector<long long> a) {
  for (int i = 0; i < a.size(); i++) {
    cout << a[i] << ' ';
  }
  cout << "\n";
}
vector<long long> inp(int n) {
  vector<long long> a;
  long long x;
  for (int i = 0; i < n; i++) {
    cin >> x;
    a.push_back(x);
  }
  return a;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long double x;
  cin >> x;
  cout << 2 * 3.14159265359 * x;
}
