#include<iostream>
#include<iomanip>
#include<vector>
#include<string>
#define rep(i, start, end) for (int i = (int)start; i < (int)end; ++i)
#define rrep(i, start, end) for (int i = (int)start - 1; i >= (int)end; --i)
#define all(x) (x).begin(), (x).end()
using namespace std;
using ll = long long;
template<typename T> inline bool chmax(T& a, T b) {if (a < b) {a = b; return true;} return 0;}
template<typename T> inline bool chmin(T& a, T b) {if (a > b) {a = b; return true;} return 0;}

const double PI=3.14159265358979323846;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    double R;
    cin >> R;
    cout << fixed << setprecision(10) << 2 * PI * R << endl;
    return 0;
}