#include<bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < (int)(n); i++)
#define all(x) (x).begin(),(x).end()
#define SZ(x) ((int)(x).size())
using namespace std;
using lint = long long;//
template <typename T>
T sq (T x) {
    return x * x;
}

signed main () {
    lint  R;
    cin >> R;
 cout <<   2. * M_PI *R << endl;


    return 0;
}

