#include <bits/stdc++.h>
using namespace std;
int dx[] = {0, 0, 1, -1};
int dy[] = {1, -1, 0, 0};
const long double Pi = acos(-1), e = 2.718;
const int N = 2e5 + 10, mod = 1e9 + 7;
int main() {
  ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  double r;
  cin >> r;
  cout << setprecision(12) << 2 * Pi * r << '\n';
  return 0;
}
