#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using pl = pair<ll, ll>;
const ll MOD = 1000000007;
const int inf = 1e9 + 10;
const ll INF = 4e18;
const ll dy[4] = {1, 0, -1, 0};
const ll dx[4] = {0, -1, 0, 1};
int main() {
  cin.tie(0);
  cout.tie(0);
  ios::sync_with_stdio(false);
  double r;
  cin >> r;
  cout << fixed << setprecision(10) << r * 2 * M_PI << endl;
  return 0;
}
