//#pragma GCC optimize ("Ofast")
//#pragma GCC optimize ("unroll-loops")
//#pragma GCC target("avx,avx2,fma")


#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/rope>
#define pb push_back
#define F first
#define S second
#define ins insert
#define mp make_pair
#define bit __builtin_popcount
#define md (l + ((r - l) / 2))
#define all(x) x.begin(),x.end()
#define eb emplace_back
#define ub upper_bound
#define lb lower_bound
#define ios ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define file(s) if (fopen(s".in", "r")) freopen(s".in", "r", stdin), freopen(s".out", "w", stdout)
#define sqr(x) (x*x)

using namespace std;
using namespace __gnu_pbds;
using namespace __gnu_cxx;


using ll = long long;

//#define int ll


const int N = 1e5+33, mod = 1e9 + 7, mod2 = 998244353;
const int MAX = 1e5 + 11;
const int INF1 = 2e9 + 11;
const ll INF2 = 2e18 + 11;
const double INF3 = 1e8 + 11;
const int base = 500;
const int P = 31;
const int dx[] = {1, -1, 0, 0, 1, 1, -1, -1};
const int dy[] = {0, 0, 1, -1, 1, -1, 1, -1};
const char pxy[] = {'R' , 'L' , 'D' , 'U'};
const double EPS = 1e-4;
const double PI = acos(-1.0);

template<typename T> using ordered_set = tree <T, null_type, less <T>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename T1, typename T2> inline void chmin(T1 &a, T2 b) { if (a > b) a = b; }
template<typename T1, typename T2> inline void chmax(T1 &a, T2 b) { if (a < b) a = b; }


typedef pair <int, int> pii;        // #include <ext/pb_ds/assoc_container.hpp>
typedef pair <ll, ll> pll;         // #include <ext/pb_ds/tree_policy.hpp>
typedef vector <int> vi;          // #include <ext/rope>
                // #include <ext/rope>
                                 // #include <ext/rope>
bool prime(int n) {             // template<typename T> using ordered_set = tree <T, null_type, less <T>, rb_tree_tag, tree_order_statistics_node_update>;
    for(int i=2;i<=sqrt(n);i++){
        if(n%i==0){return 0;}
    }return 1;}
ll bp(ll a,ll b) {ll res=1;a%=mod; assert(b>=0); for(;b;b>>=1){if(b&1)res=res*a%mod;a=a*a%mod;}return res;}

void solve() {
    double r;
    cin >> r;
    cout << 2*PI*r;
}

/*
*/

main() {
    ios;
    int tt = 1;
//    cin >> tt;
    while(tt --) {
        solve();
        cout << '\n';
    }
    return 0;
}
/*
++>
+++>
<[<+>-]<
++++++++++++++++++++++++++++++++++++++++++++++++.

500000000000000000000000000000000000000000000000

1 1 2 3 6


*/
