#include<iostream>
#include<algorithm>
#include<vector>
#include<string>
#include<utility>
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<functional>
#include<math.h>
using namespace std;
#define N (1000000000+7)
#define M (998244353)
#define INF 1e16
typedef long long ll;
typedef pair<int,int> P;



const double PI=3.14159265358979323846;

int main(void){
    ll r;
    cin>>r;
    printf("%.10lf",(double)2.0*PI*r);
    cout<<endl;
    return 0;
}