/*
@Killmonger
 
 
 
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*/
 
 
//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")
//#pragma GCC target("sse2") 
 
 
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
 
#define pb                push_back
#define ppb               pop_back
#define pf                push_front
#define ppf               pop_front
#define all(x)            (x).begin(),(x).end()
#define MOD               1000000007
#define int               long long
#define ll                long long
#define rep(i,a,b)        for(int i=a;i<b;i++)
#define repe(i,a,b)       for(int i=a;i<=b;i++)
#define repr(i,a,b)       for(int i=a;i>=b;i--)
#define sz(x)             (int)((x).size())
#define mkp               make_pair
#define pq                priority_queue
#define mpq               priority_queue<int,vector<int>,greater<int> >
#define inputarr(a,n)       for(int i=0;i<n;++i) cin>>a[i]
#define initarr(a,n,x)    for (int i=0;i<n;++i) a[i]=x
#define invect(data,n,commands)    for(int i = 0;i<n;i++){int tmp;cin>>tmp;data.pb(tmp);commands}
#define inset(data,n,commands)     for(int i = 0;i<n;i++){int tmp;cin>>tmp;data.insert(tmp);commands}
#define trav(a, x)           for(auto& a : x)
#define display(x)           trav(a,x) cout<<a<<" ";cout<<endl
#define deb(x)                   cout<<#x<<" "<<x<<endl;
#define DEBG              cout<<"I am here!"<<endl;
#define fr                first
#define sc                second
#define vc                vector
#define lb                lower_bound
#define ub                upper_bound
#define pii               pair<int,int>
#define msi               map<string,int>
#define mii               map<int,int>
#define psi               pair<string,int>
#define pis               pair<int,string>
#define gcd(a,b)          __gcd(a,b)
#define lcm(a,b)         (a*(b/gcd(a,b))) 
#define accuracy           cout << fixed << setprecision(18); 
#define endl             '\n'
#define MAX_N                 200005
#define between(n,a,b)      (a<=n&&n<=b)
#define clamp(n,a,b)     (((n)<(a))?(a):(((n)>(b))?(b):(n)))
#define mem1(a)           memset(a,-1,sizeof(a))
#define mem0(a)           memset(a,0,sizeof(a))
#define ppc               __builtin_popcount
 
const double PI  = 3.141592653589793238463;  

vector<string> vec_splitter(string s) {
    s += ',';
    vector<string> res;
    while(!s.empty()) {
        res.push_back(s.substr(0, s.find(',')));
        s = s.substr(s.find(',') + 1);
    }
    return res;
}
void debug_out(
vector<string> __attribute__ ((unused)) args,
__attribute__ ((unused)) int idx, 
__attribute__ ((unused)) int LINE_NUM) { cerr << endl; } 
template <typename Head, typename... Tail>
void debug_out(vector<string> args, int idx, int LINE_NUM, Head H, Tail... T) {
    if(idx > 0) cerr << ", "; else cerr << "Line(" << LINE_NUM << ") ";
    stringstream ss; ss << H;
    cerr << args[idx] << " = " << ss.str();
    debug_out(args, idx + 1, LINE_NUM, T...);
}
#ifdef killmonger
#define debug(...) debug_out(vec_splitter(#__VA_ARGS__), 0, __LINE__, __VA_ARGS__)
#else
#define debug(...) 42
#endif
 
 
 
inline  void normal(int &a) { a = (a+MOD)%MOD; }
inline  int modMul(int a, int b) { a %= MOD, b %= MOD; normal(a), normal(b); return (a*b)%MOD; }
inline  int modAdd(int a, int b) { a %= MOD, b %= MOD; normal(a), normal(b); return (a+b)%MOD; }
inline  int modSub(int a, int b) { a %= MOD, b %= MOD; normal(a), normal(b); a -= b; normal(a); return a; }
inline  int modPow(int b, int p) { int r = 1; while(p) { if(p&1) r = modMul(r, b); b = modMul(b, b); p >>= 1; } return r; }
inline  int modInv(int a) { return modPow(a, MOD-2); }
inline  int modDiv(int a,int b) { return modMul(a, modInv(b)); }
 
 
inline long long toint(const std::string &s) {std::stringstream ss; ss << s; long long x; ss >> x; return x;}
inline std::string tostring(long long number) {std::stringstream ss; ss << number; return ss.str();}
inline std::string tobin(long long x) {return std::bitset<63>(x).to_string();}
 
 
 
template<typename T1,typename T2>istream& operator>>(istream& in,pair<T1,T2> &a){in>>a.fr>>a.sc;return in;}
template<typename T1,typename T2>ostream& operator<<(ostream& out,pair<T1,T2> a){out<<a.fr<<" "<<a.sc;return out;}
template<typename T>pair<T,T> operator+=(pair<T,T> &a,pair<T,T> b){a.fr+=b.fr;a.sc+=b.sc;return a;}
template<typename T>pair<T,T> operator-=(pair<T,T> &a,pair<T,T> b){a.fr-=b.fr;a.sc-=b.sc;return a;}
template<typename T>pair<T,T> operator+(pair<T,T> a,pair<T,T> b){a.fr+=b.fr;a.sc+=b.sc;return a;}
template<typename T>pair<T,T> operator-(pair<T,T> a,pair<T,T> b){a.fr-=b.fr;a.sc-=b.sc;return a;}
template<typename T,typename T1>T amax(T &a,T1 b){if(b>a)a=b;return a;}
template<typename T,typename T1>T amin(T &a,T1 b){if(b<a)a=b;return a;}
template <typename Arg1>
void __f(const char* name, Arg1&& arg1){
    std::cerr << name << " : " << arg1 << endl;
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args){
    const char* comma = strchr(names + 1, ',');std::cerr.write(names, comma - names) << " : " << arg1<<" | ";__f(comma+1, args...);
}    
 
template <typename T> T allmin(T t) {return t;}
template<typename T, typename... Args> T allmin(T t, Args... args) {return min(t, allmin(args...));}
 
template <typename T> T allmax(T t) {return t;}
template<typename T, typename... Args> T allmax(T t, Args... args) {return max(t, allmax(args...));}
 
 
#define max allmax
#define min allmin
 
/*----------------------Graph Moves----------------------*/
// const int fx[]={+1, -1, +0, +0};
// const int fy[]={+0, +0, +1, -1};
// const int fx[]={+0, +0, +1, -1, -1, +1, -1, +1};  // Kings Move
// const int fy[]={-1, +1, +0, +0, +1, +1, -1, -1};  // Kings Move
// const int fx[]={-2, -2, -1, -1,  1,  1,  2,  2};  // Knights Move
// const int fy[]={-1,  1, -2,  2, -2,  2, -1,  1};  // Knights Move
/*-------------------------------------------------------*/

void km(){
    int n;
    cin>>n;
    cout<<2*PI*n;
}


 
signed main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    //ios_base& scientific (ios_base& str);
    clock_t start, end; 
    start = clock(); 
 
    accuracy;
    int testcases=1;
    //cin>>testcases;
    //freopen("input.txt","r",stdin);
    //freopen("output.txt","w",stdout);
    
    while(testcases--){
    km();
    }
 
    end = clock();
    double time_taken = double(end - start) / double(CLOCKS_PER_SEC); 
    //cerr<<time_taken;
    
 
    }
 