#include <bits/stdc++.h>
using namespace std;
#define mp make_pair
#define pb(x) push_back(x)
#define vi vector<int>
#define vs vector<string>
#define vll vector<long long>
#define vc vector<char>
#define vs vector<string>
#define pii pair<int, int>
#define pll pair<long long, long long>
#define pil pair<int, long long>
#define pli pair<long long, int>
#define pcc pair<char, char>
#define pdd pair<double, double>
#define mll map<long long, long long>
#define sz(v) ((int)(v).size())
#define all(v) (v).begin(), (v).end()
#define flsh fflush(stdout)
#define gcd __gcd
#define clr(x) memset(x, 0, sizeof(x))
#define mod 1000000007LL
#define INF 1000000000007
 

typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;


void solve()
{
    ll r;
    cin>>r;
    // cout<<r<<"\n";
    ld ans=2.0*3.14159*r;
    cout<<ans<<"\n"; 
}


int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);


if (fopen("input.txt","r" ))
  {
     freopen("input.txt", "r", stdin);
     freopen("output.txt", "w", stdout);
  }
cout<<setprecision(20);


ll t=1;


// cin>>t;


while(t--)
{
solve();
} 


return 0;
}