#include<bits/stdc++.h>
using namespace std;
#define flb(i,a,b) for(int i=a;i<b;i++)
#define ll long long int
#define mp make_pair
#define pb push_back
#define inf (long long)(1e18+1e15)
typedef pair<int,int> ii;
typedef vector <int> vi;
typedef vector <pair<int,int> > vii;
typedef vector <vector <int> > vvi;

int main()
{
    float r;
    cin>>r;
    float pi=3.1415926536;
    float ans=2*pi*r;
    printf("%f",ans);
}
