//header{{{
#pragma GCC optimize("Ofast")
#include<bits/stdc++.h>
//#include<boost/multiprecision/cpp_int.hpp>
//#include<boost/math/common_factor_rt.hpp>
//#include<boost/multiprecision/cpp_dec_float.hpp>
  
using namespace std;
//using namespace boost;
//using namespace multiprecision;  

#define rep(i,n) for(int i=0;i<(n);++i)
#define reps(i,n) for(int i=1;i<=(n);++i)
#define all(x) (x).begin(),(x).end()
#define Fixed fixed << setprecision(12)
//#define int int64_t
using pii = pair<int,int>;
constexpr int INF  = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr int mod1 = 1e9+7; 
constexpr int mod2 = 998244353;

template <class A, class B> inline bool chmax(A &a, const B &b) { return b > a && (a = b, true); }
template <class A, class B> inline bool chmin(A &a, const B &b) { return b < a && (a = b, true); }

template <class T> using min_heap = priority_queue<T,vector<T>,greater<T> >;
template <class T> using max_heap = priority_queue<T>;
template <class A, class B> using umap = unordered_map<A,B>;
  
inline int square(int a){ return a * a;}
inline int updiv(int a,int b){ return (a + b - 1) / b; }

template <class A, class B> inline int Max(const A &a, const B &b){ return (a < b ? b : a); }
template <class A, class B> inline int Min(const A &a, const B &b){ return (a > b ? b : a); }

int gcd(int a,int b){ return b ? gcd(b,a % b) : a;}

constexpr int dx[] = {1,0,-1,0,1,1,-1,-1};
constexpr int dy[] = {0,-1,0,1,1,-1,-1,1};
//}}}


signed main(){
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);

    int r;
    cin >> r;

    cout << r * 2 * M_PI << '\n';

    return 0;
}
