#include <bits/stdc++.h>
using namespace std;
long long int fact(long long int n) {
  long long int s = 1;
  for (long long int i = 1; i <= n; i++) s *= i;
  return s;
}
long long int ceil1(long long int n, long long int m) {
  if (n % m == 0) {
    return n / m;
  }
  return n / m + 1;
}
vector<long long int> factor(long long int n) {
  vector<long long int> ans;
  if (n > 0) ans.push_back(1);
  if (n == 2) ans.push_back(2);
  for (long long int i = 2; i * i <= n; i++) {
    if (n % i == 0) {
      ans.push_back(i);
      if (n / i != i) ans.push_back(n / i);
    }
  }
  if (n > 2) ans.push_back(n);
  return ans;
}
long long int nCr(long long int n, long long int r) {
  return fact(n) / (fact(r) * fact(n - r));
}
void dfs(vector<vector<long long int>> &v, long long int k,
         vector<long long int> &par, vector<long long int> &dis,
         long long int p) {
  par[k] = p;
  dis[k] = dis[p] + 1;
  for (long long int i = 0; i < v[k].size(); i++) {
    if (v[k][i] != p) {
      dfs(v, v[k][i], par, dis, k);
    }
  }
}
bool sortbysec(const pair<int, int> &a, const pair<int, int> &b) {
  return (a.second < b.second);
}
void solve() {
  double n;
  cin >> n;
  cout << 2.0 * 3.14159265358979323846 * n;
  return;
}
int main() {
  long long int q;
  q = 1;
  while (q--) {
    solve();
  }
  return 0;
}
