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

#define ll                      long long
#define pii                     pair<int, int>
#define pli                     pair<ll, int>
#define pil                     pair<int, ll>
#define pll                     pair<ll, ll>
#define vi                      vector<int>
#define vll                     vector<ll>
#define vb                      vector<bool>
#define vd                      vector<double>
#define vs                      vector<string>
#define ff                      first
#define ss                      second
#define pb                      push_back
#define eb                      emplace_back
#define ppb                     pop_back
#define pf                      push_front
#define ppf                     pop_front
#define vpii                    vector<pii>
#define umap                    unordered_map
#define all(x)                  x.begin(),x.end()
#define clr(a,b)                memset(a,b,sizeof a)
#define fr(i, n)                for(int i=0; i<n;++i)
#define fr1(i, n)               for(int i=1; i<=n; ++i)
#define rfr(i, n)			    for(int i=n-1; i>=0; --i)
#define precise(x)              cout<<fixed<<setprecision(x)
#define wh						while
#define mp						make_pair
#define le						length()
#define M						1000000007


void solve()
{
	float ans=6.28318530717958623200;
float r;
cin>>r;
cout<<ans*r<<endl;

}


int main()
{
	ios::sync_with_stdio(0); 
	cin.tie(0);
	
	solve();
}