#include <bits/stdc++.h> 
#include <Windows.h>
#include <time.h>
#define F(i,a,b) for(int i=a;i<=(b);++i)
#define dF(i,a,b) for(int i=a;i>=(b);--i)
#define inf 0x3f3f3f3f
#define infll 0x3f3f3f3f3f3f3f3f
#define pb push_back
#define maxnkp make_pair
#define fi first
#define se second
#define eps 1e-10
using namespace std;
typedef long long ll;
const int mods = 1e9+7;
const int maxn = 1e5+10;
const int N = 1e5+10;
const int E = 2e5+10;  
ll n; 

int main(){
	double r;
	cin>>r;
	r = acos(-1)*2.0*r;
	printf("%llf\n",r);
}