#include <bits/stdc++.h>
using ll = long long;
using namespace std;
#define rep(i, n) for (int i = 0; i < n; i++)
#define reprev(i, n) for (int i = n - 1; i >= 0; i--)

int main(){
  double R, answer;
  cin >> R;
  answer = 2 * R * 3.141;
  cout << answer << endl;
}
