#include <bits/stdc++.h>
#include <cstring>
#include <limits>
using namespace std;
#define lli long long int
#define X first
#define Y second
#define fw(i, start, end) for (lli i = start; i < end; ++i)
#define fe(i, start, end) for (lli i = start; i <= end; ++i)
#define pi 3.141592
#define L(T)  \
    lli T;    \
    cin >> T; \
    while (T--)
#define FASTIO                   \
    ios::sync_with_stdio(false); \
    cin.tie(nullptr);            \
    cout.tie(nullptr);
int main()
{
    FASTIO
    //   L(T)
    {
       double n;
        cin>>n;
        cout<<2*pi*n<<"\n";
    }
    return 0;
}