#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

#define INF (long long)(1e9+7)
#define REP(i,n) for(ll i = 0; i < n; i++)
#define REP1(i,n) for(ll i = 1; i <= n; i++)

int main(void)
{
  ll r;
  cin >> r;
  printf("%.7f\n", r*2*3.1415926);
  
}
