//aea ctmr
#pragma GCC optimize("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include <bits/stdc++.h>
//v.erase(unique(v.begin(),v.end()),v.end());
#define f(i,a,b) for( ll i = a; i < b ; i++ ) 
#define af(i,a,b) for( ll i = a; i >= b ; i--)
#define rep(i,a,b,k) for(ll i = a; i < b ; i+= k )
#define arep(i,a,b,k) for( ll i = a; i >= b ; i-= k)
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
#define sor(a) sort( a.begin(), a.end() )
#define fastio ios_base::sync_with_stdio(false);cin.tie(NULL)
 
using namespace std;
 
typedef long double ld;
typedef long long ll;
typedef  pair <ll, ll> pii;
 
const ll N = 200500;
const ll mod = 1e9 + 7;
const ld PI = acos(-1); 
 
#define TRACE
#ifdef TRACE
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
  cerr << name << " : " << arg1 << endl;}
 
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
  const char* comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << " : " << arg1<<" | ";
  __f(comma+1, args...);}
#else
#define trace(...)
#endif
 


int main() {
	fastio;

	ld n;
	cin >> n;

	cout << (ld)2.0 * n * PI << endl;
	
	return 0;
}
