#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
 
using namespace __gnu_pbds;
using namespace std;
 
typedef   long long int ll;
typedef unsigned long long int sll;
typedef  double ld;
#define A 1000000007ll
#define B 998244353ll
#define C 1000000000000000000ll
#define FAST ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define pb push_back
#define mxe(v) *max_element(v.begin(),v.end())
#define mne(v) *min_element(v.begin(),v.end())
#define bs binary_search
#define lb lower_bound
#define ub upper_bound
#define ve vector
#define br break
#define PI 3.141592653

int main() 
{
    FAST;
    long double r,c;
    cin>>r;
    c=PI*2.0*r;
    cout.precision(12);
    cout<<c;
    return 0;
}