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

#define lli long long int
#define pb push_back
#define in insert
#define pi pair<int, int>
#define pd pair<double, int>
#define pii pair<int, pi>
#define mp make_pair
#define fir first
#define sec second
#define MAXN 200001
#define mod 1000000000

signed main()
{
  lli n;
  cin >> n;
  float ans = 2 * 3.1415 * n;
  cout << ans << endl;
  return 0;
}