#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep_range(i, s, e) for (int i = (s); i < (int)(e); i++)
#define rep_rev(i, s, e) for (int i = (s)-1; i >= (int)(e); i--)
#define sort_vec(v) sort((v).begin(),(v).end())
typedef int64_t lint;
// lint mod = 998244353;

double app() {
  int a;
  cin>>a;

  return a*2*3.14159265;
}

int main(){cout<<app()<<endl;return 0;}
