//@author yash2040
#include<bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
#define ll long long
#define int long long
#define pb push_back
#define mod 1000000007
#define fast() ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
main()
{
	fast();
	int r;
	cin>>r;
	double ar=2*acos(-1.0)*r;
	cout<<ar<<endl;	
}



