#include "bits/stdc++.h"
 
using namespace std;
 
#define fast_cin() ios_base::sync_with_stdio(false);cin.tie(NULL)
#define rep(i, x, n) for(int i=x; i<n; i++)
#define all(x) x.begin(), x.end()
#define pb push_back
#define eb emplace_back
#define trav(a, x) for(auto& a: x)
#define fi first
#define se second
#define lb lower_bound
#define ub upper_bound
#define MOD 1000000007
#define endl '\n'
#define MAX 100002
#define pii pair<int, int>
#define pll pair<long long, long long>
 
typedef vector<int> vec;
typedef long long int ll;
typedef unsigned long long ull;



int main(){
    fast_cin();
    double n;cin>>n;
    double ans=2*n*3.14;
    printf("%.20f\n",ans);
}