#include <bits/stdc++.h>
using namespace std;
const int dx[] = {-1, -1, -1, 0, 0, 0, 1, 1, 1};
const int dy[] = {-1, 0, 1, -1, 0, 1, -1, 0, 1};
void AIA() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
}
int main() {
  cout << fixed << setprecision(20);
  AIA();
  long double n;
  cin >> n;
  cout << n * 6.283185307179586232;
  return 0;
}
