#include <bits/stdc++.h>
const double PI = 3.141592653589793238463;
using namespace std;
const int MAXN = 1000500;
long long int mod = 1000000009;
long long int T, B;
long long int s_left = -1;
long long int d_left = -1;
long long int n, m, r;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.precision(16);
  cin >> r;
  cout << 2 * r * PI << endl;
  return 0;
}
