#include <limits>
#include <initializer_list>
#include <utility>
#include <bitset>
#include <tuple>
#include <type_traits>
#include <functional>
#include <string>
#include <array>
#include <deque>
#include <list>
#include <queue>
#include <stack>
#include <vector>
#include <map>
#include <set>
#include <unordered_map>
#include <unordered_set>
#include <iterator>
#include <algorithm>
#include <complex>
#include <random>
#include <numeric>
#include <iostream>
#include <iomanip>
#include <sstream>
#include <regex>
#include <cassert>
#include <cstddef>
#define endl codeforces
#define ALL(v) std::begin(v), std::end(v)
#define ALLR(v) std::rbegin(v), std::rend(v)
using ll = std::int64_t;
using ull = std::uint64_t;
using pii = std::pair<int, int>;
using tii = std::tuple<int, int, int>;
using pll = std::pair<ll, ll>;
using tll = std::tuple<ll, ll, ll>;
template <typename T> using vec = std::vector<T>;
template <typename T> using vvec = vec<vec<T>>;
template <typename T> const T& var_min(const T &t) { return t; }
template <typename T> const T& var_max(const T &t) { return t; }
template <typename T, typename... Tail> const T& var_min(const T &t, const Tail&... tail) { return std::min(t, var_min(tail...)); }
template <typename T, typename... Tail> const T& var_max(const T &t, const Tail&... tail) { return std::max(t, var_max(tail...)); }
template <typename T, typename... Tail> void chmin(T &t, const Tail&... tail) { t = var_min(t, tail...); }
template <typename T, typename... Tail> void chmax(T &t, const Tail&... tail) { t = var_max(t, tail...); }
template <typename T> const T& clamp(const T &t, const T &low, const T &high) { return std::max(low, std::min(high, t)); }
template <typename T> void chclamp(T &t, const T &low, const T &high) { return t = clamp(t, low, high); }
template <typename T> T make_v(T init) { return init; }
template <typename T, typename... Tail> auto make_v(T init, std::size_t s, Tail... tail) { auto v = std::move(make_v(init, tail...)); return vec<decltype(v)>(s, v); }
template <typename T, std::size_t Head, std::size_t ...Tail> struct multi_dem_array { using type = std::array<typename multi_dem_array<T, Tail...>::type, Head>; };
template <typename T, std::size_t Head> struct multi_dem_array<T, Head> { using type = std::array<T, Head>; };
template <typename T, std::size_t ...Args> using mdarray = typename multi_dem_array<T, Args...>::type;
namespace init__ { struct InitIO { InitIO() { std::cin.tie(nullptr); std::ios_base::sync_with_stdio(false); std::cout << std::fixed << std::setprecision(30); } } init_io; }

namespace geo {

using value_type = long double;
using Point = std::complex<value_type>;

const value_type eps = 1e-12;
const value_type pi = 3.14159265358979323846264338327950288419716939937510L;

const value_type err_v = std::numeric_limits<value_type>::max();
const Point err_p(err_v, err_v);

struct DualPoints : public std::pair<Point, Point> {
    using std::pair<Point, Point>::pair;

    Point& operator [](size_t idx) { return idx == 0 ? first : second; }
    
    const Point& operator [](size_t idx) const { return idx == 0 ? first : second; }
};

struct get_xy {
    value_type &x, &y;

    get_xy(value_type &x, value_type &y) : x(x), y(y) { }

    get_xy& operator =(const Point &p) {
        x = std::real(p);
        y = std::imag(p);
        return *this;
    }
};

struct Circle {
    Point p;
    value_type r;

    Circle(Point p, value_type r) : p(p), r(r) { }
};

struct Line : public DualPoints {
    using DualPoints::DualPoints;
};

struct Seg : public DualPoints {
    using DualPoints::DualPoints;
};

value_type dot(const Point &a, const Point &b) {
    return (std::conj(a) * b).real();
}

value_type cross(const Point &a, const Point &b) {
    return (std::conj(a) * b).imag();
}

bool is_zero(value_type a) {
    return std::abs(a) <= eps;
}

bool eq(value_type a, value_type b) {
    return is_zero(a - b);
}

// projection from p to l
Point proj(const Line &l, const Point &p) {
    value_type mul = dot(p - l[0], l[1] - l[0]) / norm(l[0] - l[1]);  // cos(theta) / (p-l[0]).norm()
    return l[0] + mul * (l[1] - l[0]);
}

Point refl(const Line &l, const Point &p) {
    Point pr = proj(l, p);
    return p + ((pr - p) * static_cast<value_type>(2));
}

value_type fix_arg(value_type arg) {
    while (arg < 0) arg += 2 * pi;
    while (2 * pi <= arg) arg -= 2 * pi;
    return arg;
}

value_type fix_arg(const Point &a) {
    return fix_arg(std::arg(a));
}

bool comp_coclock(const Point &p1, const Point &p2) {
    return fix_arg(arg(p1)) < fix_arg(arg(p2));
}

Point rotate(const Point &p, value_type arg) {
    std::complex<value_type> r(std::cos(arg), std::sin(arg));
    return p * r;
}

value_type cos(value_type a, value_type b, value_type c) {
    return (a * a + b * b - c * c) / (2 * a * b);
}

bool is_parallel(const Line &l1, const Line &l2) {
    return is_zero(cross(l1[1] - l1[0], l2[1] - l2[0]));
}

bool is_same_line(Point o, Point a, Point b) {
    return is_zero(cross(a - o, b - o));
}

}

int main() {
    long double a;
    std::cin >> a;
    std::cout << 2 * a * geo::pi << '\n';
    return 0;
}


Measured memory_bytes: 3820