#include <bits/stdc++.h>
#include <iomanip>
#define ll long long
#define pb push_back
#define mp make_pair
#define mt make_tuple
#define vi vector <ll> 
#define qu queue <ll>
#define min_3(a,b,c) min(a,min(b,c))
#define max_3(a,b,c) max(a,max(b,c))
#define nout(ans) cout << (ans) << "\n"
#define sout(ans) cout << (ans) << " "
using namespace std;

/*
Decimal Places: cout << fixed << setprecision(n) << ans << endl;
*/

bool compare(const pair<int, int>&i, const pair<int, int>&j)
{
    return i.second < j.second;
}


void solve()
{
    float r; cin >> r;
    nout(2*r*3.1415);
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL); cout.tie(NULL);
    int t = 1;
    //cin >> t;
    while (t--)
    {
        solve();
    }
    return 0;
}