#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>

using namespace std;
using namespace __gnu_pbds;
using ll = long long;

#define     _test int _TC; cin >> _TC; while(_TC--)
#define     CS(x) cout << "Case " << x << ": ";
#define     precision(n) cout << fixed << setprecision(n);
#define     Clear(a, b) memset(a, b, sizeof(a))
#define     sq(x) ((x) * 1LL * (x))
#define     eb emplace_back
#define     all(_n) _n.begin(), _n.end()
#define     rall(_n) _n.rbegin(), _n.rend()
#define     endl "\n"
#define     show(x) cout << x << endl
#define     nl cout << endl
#define     sz(x) int(x.size())
#define     gcd(a, b) __gcd(a, b)
#define     lcm(a, b) (abs((a) * (b)) / gcd(a, b))
#define     parity(a, b) ((a&1) ^ (b&1))
#define     make_unique(x) sort(all((x))); (x).erase(unique(all((x))), (x).end())
#define     IN(a) for(auto &it : a) cin >> it

template <typename T> void print (T t){ for(const auto &it : t) cout << it << ' '; nl; }
template <typename T> using OS = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>; // order_of_key find_by_order

const    double Pi = acos(-1.0);
const    int Mod = 1e9 + 7;
const    int Inf = 1 << 30; // INT_MAX = (1LL << 31) -1 // Inf -> 1e9
const    int Maxx = 1e6 + 10;

/************************************[Code Starts Here]************************************/


void _case()
{
	int n; cin >> n;
	precision(2)
	show(2*Pi*n);
}

int32_t main()
{
    ios::sync_with_stdio(false), cin.tie(nullptr);
    _case();
    return 0;
}


