#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;
 
#define ll long long
#define uint unsigned int
#define ull  unsigned long long
#define pb push_back
#define mk make_pair
#define ins insert
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define X first
#define Y second
#define umap unordered_map
#define speed() ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define setvalue(d,s,e,n) for(int qwe = s; qwe < e; ++qwe) d[qwe] = n
#define mset multiset
#define pqueue priority_queue
   
template<class T> using oset=tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
        
const int  N = 2e5 + 314;
const int INF = 1e9;
const double PI = acos(-1);
const int MOD = 1e9 + 7;
const double eps = 1e-9;
 
void solve(){
    int r;
    cin >> r;
    cout <<fixed << setprecision(10)<< 2.0L * PI * r; 
}
 
int main(){
    speed();
    int t = 1;
    // cin >> t;
    while(t--)solve();
}