#include <bits/stdc++.h>
using namespace std;
using LL = long long;
using LD = long double;
using pii = pair<long long, long long>;
using vii = vector<pii>;
const long long INF = 1e18;
const long long MOD = 1e9 + 7;
const long long N = 1e5 + 5;
int32_t main() {
  double r;
  cin >> r;
  double pi = acos(-1);
  cout << fixed << setprecision(10);
  cout << 2 * pi * r << "\n";
}
