#include<bits/stdc++.h>
typedef long long ll;
typedef long double ld;
#define FASTIO ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define PRECISION std::cout << std::fixed << std::setprecision(20);
#define setzero(x) memset(x,0,sizeof(x))
  
using namespace std;
 
const int inf = 2e9+500;
const int N = 1e3+10;
const int bits = 20;
const ld pi = 3.14159265358979323846;
const ll mod = 1e9;
const ll p = 29;


int main(){

    FASTIO;
    PRECISION; 
    ld r;
    cin>>r;
    cout<<2.0*pi*r<<"\n";
    

}