AtCoderテンプレ
#include <bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < (n); ++i)
using namespace std;
using ll = long long;
using P = pair<int,int>;

int main(){
  double ans;
  cin >> ans;
  cout << ans*2*3.14159265 < endl;
}
