#include<bits/stdc++.h>
using namespace std;

#define ll long long int
#define PI acos(1.0)

#define Bit_off(val,pos)       (val & (~(1<<pos)))
#define Bit_on(val,pos)        (val | (1<<pos))
#define Bit_toggle(val,pos)    (val ^ (1<<pos))
#define Bit_check(val,pos)     (bool)(val & (1<<pos))
#define odd_even(val)          (bool)(val & 1)
#define koita_1_ase(val)       (__builtin_popcount(val))

ll ar[500009],br[500009],cr[500009];
map<ll,ll>mp,mp1;
vector<ll>v;
string s,d;

int main()
{
    ll a,b,c,i,j,k,x,y,z,f,g,h;
    
    double q,w,e,r;
    
    cin>>q;
    
    r=(2.0*q*PI);
    
    cout<<fixed<<setprecision(6)<<r<<endl;
}

