#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define ull unsigned long long
#define dl double long
const int INF=0x3fffffff;
const ll MOD=1000000007;
const ll MODD=998244353;
#define rrep(i,n) for(ll i=(ll)(n)-1;i>=0;i--)
#define REP(i,m,n) for(ll i=(ll)(m);i<(ll)(n);i++)
#define rep(i,n) REP(i,0,n)
template<typename T> bool chmax(T &m, const T q) { if (m < q) {m = q; return true;} else return false; }
template<typename T> bool chmin(T &m, const T q) { if (m > q) {m = q; return true;} else return false; }

int main()
{
  dl R;
  cin >> R;
  cout << 2*3.14*R << "\n";
}