#include <bits/stdc++.h>
using namespace std;
void solve();
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  time_t start, end;
  time(&start);
  solve();
  time(&end);
  double t = double(end - start);
  return 0;
}
void solve() {
  int r;
  cin >> r;
  cout << 2 * M_PI * r << "\n";
}
