#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using PII = pair<ll, ll>;
#define FOR(i, a, n) for(ll i=(ll)a; i<(ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(x) x.begin(), x.end()
template<typename T> void chmin(T &a, const T &b) { a = min(a, b); }
template<typename T> void chmax(T &a, const T &b) { a = max(a, b); }
struct FastIO {FastIO() { cin.tie(0); ios::sync_with_stdio(0); }}fastiofastio;
#ifdef DEBUG
#include "../program_contest_library/memo/dump.hpp"
#else
#define dump(...)
#endif
constexpr ll INF = 1LL<<60;

int main() {
    ll r;
    cin >> r;
    cout << fixed << setprecision(9) << 2 * r * acos(-1) << endl;

    return 0;
}
