#include <iostream>
#include<string>
#include<cmath>
#include<algorithm>
#include<cctype>
#include<queue>
#include<deque>
#include<regex>
#include<stack>
#include<stdio.h>
#include<vector>
#include<set>
#include<map>
#include<iomanip>
#define rep(i,n) for(int i=0;i<n;i++)

typedef int long long ll;
using namespace std;
typedef pair<int,int> P;
int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};
const ll MOD=1e9+7;
static const int MAX = 100;
static const int INF = (1<<23);
template<class T> T gcd(T a, T b){return b? gcd(b,a%b) : a;}
template<class T> T lcm(T a,T b){return a / gcd(a,b)*b;}

int main(){
    cout<<setprecision(16);
    double r;
    cin>>r;
    cout<<2*r*M_PI<<endl;






return 0;
}
