#include <bits/stdc++.h>
#define ll long long int
#define N 100
#define f(i, a, b) for (ll i = (ll)a; i <= (ll)b; i++)
#define rf(i, a, b) for (ll i = (ll)a; i >= (ll)b; i--)
#define po pop_back
#define pb push_back
#define lb lower_bound
#define fi first
#define se second
#define debug(x) cout << #x << " = " << x << endl
#define ub upper_bound
#define ibs ios_base::sync_with_stdio(false)
#define cti cin.tie(0)
#define all(x) x.begin(), x.end()
#define PI 3.141592653
#define cot cout.tie(0)
#define endl "\n"
using namespace std;
/**
        * @author :: Sawlani
        *
        */

 

int main()
{
        #ifndef ONLINE_JUDGE
        freopen("input.txt","r",stdin);
        #endif
        double r;
        cin >> r;
        cout << fixed <<  setprecision(10) ;
         double ans = 2.0*PI*r;

         cout << ans;
}