#include <bits/stdc++.h>
using namespace std;
int main(){
int N;
int counter = 0;
cin >>  N;
for(int i = 0;i < N/3 + 1;i++){
if((N - 3*(i + 1) >= 0)){
counter ++;}
else  cout << counter << endl;}
}
//#include<bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<cmath>
#include<vector>
#include<queue>
#include<set>
#include<map>

/*
#include<boost/multiprecision/cpp_int.hpp>
#include <boost/multiprecision/cpp_dec_float.hpp>
#include <boost/rational.hpp>
namespace mp = boost::multiprecision;
using Real = mp::number<mp::cpp_dec_float<1024>>;
using Bint = mp::cpp_int;
*/

using namespace std;

#define int long long
#define REP(i,s,e) for((i)=(s);(i)<(e);(i)++)
#define RREP(i,s,e) for((i)=((s)-1);(i)>=(e);(i)--)
#define FOR(i,n) for((i)=(0);(i)<(n);(i)++)
#define RFOR(i,n) for((i)=((n)-1);(i)>=(0);(i)--)
#define MOD 1000000007


signed main(){
    double R;
    cin>>R;
    printf("%.12f\n",2*R*acos(-1));
}
