#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  double t, r = 1, r1 = 0, r2 = 0, k = 0, a, b, c = 1, m, d = 0, n, e, f, x = 0,
            g, p = 0, q = 0, y = 0, z = 0;
  vector<long long> v;
  vector<long long> u;
  set<long long> s;
  std::vector<int>::iterator it;
  string s1, s2, s3, s4;
  cin >> n;
  m = 2 * acos(-1) * n;
  cout << setprecision(20) << fixed << m << endl;
}
