#include<cstdio>
#include<iostream>
#include<cstring>
#include<string>
#include<cmath>
#include<algorithm>
#include<vector>
#include<queue>
#include<stack>
#include<map>
using namespace std;

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define all(v) v.begin(), v.end()

int main(){
  int r;
  cin >> r;
  double pi = acos(-1);
  cout << 2*r*pi << endl;
}