#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using P = pair<ll,ll>;
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)
#define rep2(i,m,n) for(int (i)=(m);(i)<(n);(i)++)
#define ALL(obj) (obj).begin(), (obj).end()
#define rALL(obj) (obj).rbegin(), (obj).rend()
const ll INF = 1LL<<60;
const ll MOD = 1e9+7;



				
int main() {
 	ios::sync_with_stdio(false);
  			cout.tie(nullptr);
  			cin.tie(nullptr);

  			int r;
  			cin >> r;
  			double cir = 2 * acos(-1) * (double) r;
  			cout << fixed << setprecision(10) << cir;
}