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

typedef long long ll;

#define fastio ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define forr(i,m,n) for(ll i=m;i<n;i++)
#define testcase int t;cin>>t;while(t--)
#define all(v) v.begin(),v.end()
#define rev(v) v.rbegin(),v.rend()
#define pb push_back
#define endl "\n"
#define print(v) forr(i,0,v.size())cout<<v[i]<<" ";cout<<endl;
#define print_map(m) for(auto it=m.begin(); it!=m.end();it++) cout<<(*it).first<<" "<<(*it).second<<endl;
#define vi vector<int>
#define vp vector< pair<int, int> >

ll maxi(ll a, ll b){
	if(a>b) return a;
	return b;
}

ll mini(ll a, ll b){
	if(a>b) return b;
	return a;
}

bool sorter(pair<int, int> a, pair<int, int> b){
	if(a.second!=b.second) return a.second>b.second;
	return a.first>b.first;
}

int main(){
	fastio
	ll r; cin>>r;
	cout<<3.14159*2*r<<endl;
	return 0;
}