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

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)
#define ALL(a) (a).begin(),(a).end()
#define SORT(V) sort((V).begin(),(V).end())

int main(){
  double R;
  cin >> R;
  cout << R*2*3.14 << endl;

  return 0;
}
