#include<bits/stdc++.h>
using namespace std;
#define ll long long int
#define ld long double
#define vi vector<ll>
#define vvi vector<vi >
#define pi pair<ll,ll>
#define vvpi vector<vector<pi > >
#define vpi vector<pi >
#define vppi vector<pair<ll,pi > >
#define vs vector<string>
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
#define pb push_back
#define eb emplace_back
#define mkp make_pair
#define fs first
#define se second
#define fast ios_base::sync_with_stdio(false)
#define cintie cin.tie(NULL)
int main()
{
	ll r;
	cin>>r;
	cout<<fixed<<setprecision(30)<<2*M_PI*r<<endl;
}
