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

int main()
{
  double ensyu;
  ll r;
  cin >> r;
  ensyu = r * 6.28318530717958623200;
  cout << ensyu << endl;
  return 0;
}  
  