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

#define ll long long
#define mii make_pair<int, int>
#define mll make_pair<ll, ll>
#define pii pair<int, int>
#define pll pair<ll, ll>
#define rep(i, a, b) for (int i = a; i < b; ++i)
#define vi vector<int>
#define vvi vector<vector<int>>
#define vl vector<ll>
#define vvl vector<vector<ll>>
#define vb vector<bool>
#define vvb vector<vector<bool>>
#define all(v) (v).begin(), (v).end()

int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    double pi = 3.14159265358979323846;
    int r;
    cin >> r;
    printf("%.4lf", 2 * pi * r);
}