#include <bits/stdc++.h>
using namespace std;

#define FASTIO  ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define ll long long int
#define ld long double
#define vll vector<ll>
#define pll pair<ll,ll>
#define vl(p) vector<p>
#define w while
#define For(i,s,x) for(i=s;i<x;i++)
#define Fore(i,s,x) for(i=s;i<=x;i++)
#define FOR(i,x) for(i=0;i<x;i++)
#define Rof(i,s,x) for(i=s;i>=x;i--)
#define all(v) v.begin(),v.end()
#define it(r,v) for(auto r=v.begin();r!=v.end();r++)
#define pb push_back
#define in insert
#define sz size()
#define F first
#define S second
#define nl cout<<"\n"
#define pr(a) cout<<a<<" "
#define pr2(a,b) cout<<a<<" "<<b<<"\n"
#define pr3(a,b,c) cout<<a<<" "<<b<<" "<<c<<"\n"
#define mod 1000000007

int main()
{
	FASTIO;
	int r;
	cin>>r;
	ld a;
	a=2*3.141592653589793238*r;
	cout<<a<<endl;

}
