#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i)
using namespace std;
using ll = long long;
using P = pair<int, int>;

int main() {
  int r;
  cin >> r;
  
  cout << fixed << setprecision(10) << 3.141592 * r * 2 << endl;
}