#include<iostream>
#include<vector>
#include<algorithm>
#include<iomanip>
#include<string>
#include<queue>
#include<utility>
#include<cmath>
#include<sstream>
#include<istream>
#include<numeric>

using namespace std;

int main()
{
  long long R;
  cin >> R;

  cout << 2*3.141592*R << endl;

  return 0;
}
