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

#define ld			long double
#define int         long long
#define pb          push_back
#define pii         pair<int,int>
#define vi          vector<int>
#define vii         vector<pii>
#define mi          map<int,int>
#define mii         map<pii,int>
#define all(a)      (a).begin(),(a).end()
#define x           first
#define y           second
#define sz(x)       (int)x.size()
#define endl        '\n'
#define MOD         100000000
#define sd(t) 		scanf("%d",&(t))
#define rep(i,a,b)  for(int i=a;i<b;i++)
#define hii 		cout << "hii" << endl
#define okay 		cout << "okay" << endl
#define hey 		cout << "hey" << endl
#define M_PIl       3.141592653589793238462643383279502884L 

template <class T> ostream& operator << (ostream &os, const vector<T> &v) { for (T i : v) os << i << ' '; return os; }
template <class T> ostream& operator << (ostream &os, const set<T> &v) { for (T i : v) os << i << ' '; return os; }
template <class T, class S> ostream& operator << (ostream &os, const pair<T, S> &v) { os << v.first << ' ' << v.second; return os; }
template <class T, class S> ostream& operator << (ostream &os, const unordered_map<T, S> &v) { for (auto i : v) os << '(' << i.first << "=>" << i.second << ')' << ' '; return os; } 
 
 
#ifndef ONLINE_JUDGE
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
    template <class Arg1> void __f(const char* name, Arg1&& arg1) { cerr << name << " : " << arg1 << endl; }
    template <class Arg1, class... Args>
    void __f(const char* names, Arg1&& arg1, Args&&... args) {
        const char* sep = strchr(names + 1, ',');
        cerr.write(names, sep - names) << " : " << arg1 << "  ";
        __f(sep + 1, args...);
    }
#else
#define trace(...) 
#pragma GCC optimize ("O3")
#pragma GCC optimiz1e ("unroll-loops")
#pragma GCC target("avx2,sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#define _CRT_SECURE_NO_WARNINGS
#endif ifndef ONLINE_JUDGE

void fast()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    
}

// clock_t time_p=clock();
// void starboy1299()
// {
//     time_p=clock()-time_p;
//     cerr<<"Time Taken : "<<(float)(time_p)/CLOCKS_PER_SEC<<"\n";
// }


void solve()
{	
	int r;
	cin >> r;
	ld ans = 2*(M_PI)*r;

	cout << fixed << setprecision(16) << ans << endl;
}				

int32_t main(){
	fast();
    int t = 1;
    // cin>> t;
    for(int i=1; i<=t; i++){
    	// cout << "Case #" << i << ": ";
    	solve();
    }
}


