#include <iostream>
#include <complex>
#include <vector>
#include <string>
#include <algorithm>
#include <cstdio>
#include <numeric>
#include <cstring>
#include <ctime>
#include <cstdlib>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <iomanip>
#include <list>
#include <cmath>
#include <bitset>
#include <cassert>
#include <queue>
#include <stack>
#include <deque>
#include <random>
typedef long long int ll;
typedef long double ld;
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
const ll MOD=1000000007;
const ll MAX_N=500010;
const ll INF=9999999999;

int main(){
    ll r;
    ld pi=3.1415926535;
    cin>>r;
    cout<<setprecision(10)<<2*pi*r;
    return 0;
}