#include <bits/stdc++.h>
using namespace std;
vector<long long int> v;
map<long long int, long long int> u;
void Y() {
  cout << "Yes"
       << "\n";
}
void N() {
  cout << "No"
       << "\n";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int n, m, i, j, k, p, q, o, l, s, t, z = 0, r;
  string a, b, c;
  double x, y;
  cin >> n;
  x = (double)n;
  cout << fixed << setprecision(2) << 2 * acos(-1) * x << "\n";
  return 0;
}
