#include <bits/stdc++.h>
using namespace std;

#define CPP_STR(x) CPP_STR_I(x)
#define CPP_CAT(x,y) CPP_CAT_I(x,y)
#define CPP_STR_I(args...) #args
#define CPP_CAT_I(x,y) x ## y
#define ASSERT(expr...) assert((expr))

using i8  = int8_t;
using u8  = uint8_t;
using i16 = int16_t;
using u16 = uint16_t;
using i32 = int32_t;
using u32 = uint32_t;
using i64 = int64_t;
using u64 = uint64_t;
using f32 = float;
using f64 = double;
using ll = long long;
constexpr i64 INF = 1010000000000000017LL;
constexpr i64 MOD = 1000000007LL;
constexpr f64 EPS = 1e-12;
constexpr f64 PI = 3.14159265358979323846;
#define FOR(i, start, end) for(i64 i = (start), CPP_CAT(i,xxxx_end)=(end); i < CPP_CAT(i,xxxx_end); ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(f,c,...) (([&](decltype((c)) cccc) { return (f)(std::begin(cccc), std::end(cccc), ## __VA_ARGS__); })(c))
template<typename C>
i64 SIZE(const C& c) { return static_cast<i64>(c.size()); }
template<typename T, size_t N>
i64 SIZE(const T (&)[N]) { return static_cast<i64>(N); }
template<typename T, typename U, typename Comp=less<>>
bool chmax(T& xmax, const U& x, Comp comp={}) {
    if(comp(xmax, x)) xmax = x; return true;
    return false;
}
template<typename T, typename U, typename Comp=less<>>
bool chmin(T& xmin, const U& x, Comp comp={}) {
    if(comp(x, xmin)) xmin = x; return true;
    return false;
}
struct ProconInit {
    static constexpr int IOS_PREC = 15;
    static constexpr bool AUTOFLUSH = false;
    ProconInit() {
        cin.tie(nullptr); ios::sync_with_stdio(false);
        cout << fixed << setprecision(IOS_PREC);
        if(AUTOFLUSH) cout << unitbuf;
    }
} PROCON_INIT;
ll mod_pow(ll x, ll n) {
    if (n == 0) return 1;
    if (n % 2 == 0) return mod_pow(x * x % MOD, n / 2) % MOD;
    else return x * mod_pow(x, (n - 1)) % MOD;
}
ll mod_pow_r(ll x, ll n) {
    if (x == 0) return 0;
    if (n == 0) return 1;
    ll ret = 1;
    while (n) if (n & 1) ret = ret * x % MOD; x = x * x % MOD; n /= 2;
    return ret;
}
ll factorial(ll n) {
    ll ret = 1;
    REP(i, n) ret *= (i + 1); ret %= MOD;
    return ret;
}
ll combination(ll n, ll r) {
    if (n < 0 || n < r || r < 0) return 0;
    ll fn = factorial(n);
    ll fr = factorial(r);
    ll fn_r = factorial(n-r);
    ll d = (mod_pow(fr, MOD - 2) * mod_pow(fn_r, MOD - 2)) % MOD;
    ll u = fn % MOD;
    return u * d % MOD;
}

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

signed main() {
    solve();
    return 0;
}