#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

using ll = long long;
using ull = unsigned long long;
using vl = vector<ll>;
using vi = vector<int>;
using vchar = vector<char>;
using vstr = vector<string>;
using pll =  pair<ll,ll>;

const ll mod = 1000000007;
const ll inf = 1000000000000000;

#define pi acos(-1)
#define pb push_back
#define test int TEST;cin>>TEST;while(TEST--)
#define vsort(xx) sort(xx.begin(),xx.end())
#define vrevsort(xx) sort(xx.rbegin(),xx.rend())


int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);cout.tie(NULL);
    int r;cin>>r;
  cout<<2*pi*r;
   
    return 0;
}


