#include <iostream>
#include <algorithm>
#include <vector>
#include <cstdio>
#define rep(i,n) for(int i=0;i<n;++i)
#define rep1(i,n) for(int i=1;i<=n;++i)
using namespace std;
template<class T>bool chmax(T &a, const T &b) { if(a < b){ a = b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if(a > b){ a = b; return 1; } return 0; }
int main()
{
  double r; cin >> r;
  double pi2 = 6.28318530717958623200;
  
  printf("%.7f\n", pi2*r);
  return 0;
}
