#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef vector<ll> V;
#define rep(cnt,e) for(ll cnt=0;cnt<e;cnt++ )
typedef pair<ll,ll> P;
const ll mod = 1e9+7;
const ll INF = INT64_MAX;


ll i,j,k;


int main(){
    ll N;
    cin>>N;
    cout<<2*N*M_PI<<endl;
}
