/* Author: Rennan Rocha
 * Time: $%Y%$-$%M%$-$%D%$ $%h%$:$%m%$:$%s%$
**/

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

#define F first
#define S second
#define PB push_back
#define MP make_pair

#define ll long long
#define vi vector<int>
#define vll vector<ll>
#define pi pair<int,int>
#define pll pair<ll, ll>
#define vpi vector<pi>
#define vpll vector<pll>

using namespace std;
using namespace __gnu_pbds;

typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> ordered_set;
//*find_by_order(); order_of_key();

int main() {
    ios_base::sync_with_stdio(false);

    long double r;
    cin >> r;

    cout << 2 * 3.14 * r << endl;
}
