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

int main(){
  int r;
  cin >>r;
  double ans = 2*r*3.141592653;
  cout << fixed << setprecision(6) << ans << endl;
  return 0;
}