#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<vector<pair<int, int>>> vvpp;
typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef vector<pair<int, int>> vpp;
typedef vector<string> vs;
typedef unordered_set<int> si;
typedef pair<int, int> pp;

#define all(n) (n).begin(), (n).end()
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)

template <typename Arg1>
void out(Arg1&& arg1) {
    cout << arg1 << '\n';
}

template <typename Arg1, typename ...Args>
void out(Arg1&& arg1, Args&& ...args) {
    cout << arg1 << ' ';
    out(args...);
}


template <typename T>
void outarr(vector<T> arr) {
    for (int i = 0; i < arr.size() -1; ++i) cout << arr[i] << ' ';
    cout << arr.back() << '\n';
}

template <typename T>
void printV(vector<T> A) {
    if (A.empty()) cerr << "EMTPY\n";
    else { 
        cerr << "[";
        for (int i = 0; i < A.size() -1 ; ++i) {
            cerr << A[i] << ", ";
        }
        if (A.size()) cerr << A.back() << "]\n";
    }
}


template <typename T>
void printVV(vector<vector<T>> A) {
    cerr << "[";
    for (auto v : A) {
        printV<T>(v);
    }
    cerr << "]";
}

template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
    cerr << name << ": " << arg1 << endl;
}

template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&& ...args) {
    const char* comma = strchr(names + 1, ',');
    cerr.write(names, comma - names) << ": " << arg1 << " |";
    __f(comma + 1, args...);
}

int main(int argc, char **argv) {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int R; cin >> R;
    cout << M_PI * 2 * R << endl;
    return 0;
}

