#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
const ll N = 1e5 + 123, MOD = 1e9 + 7, oo = 1e9 + 9, INF = 1e18 + 9;
const double EPS = 1e-9, PI = acos(-1);
const int dx[] = {1, -1, 0, 0, 1, 1, -1, -1},
          dy[] = {0, 0, 1, -1, 1, -1, -1, 1};
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
  int r;
  cin >> r;
  cout << 2.0 * PI * r;
  return 0;
}
