#include<bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef long int lt;
typedef vector<ll> vi;
typedef vector<int> vin;
typedef pair<ll,ll> pll;
typedef pair<int,int> pin;
#define all(a) a.begin(),a.end()
#define rep(i,a,b) for(int i=a;i<b;++i)
#define f(i,b) for(int i=0;i<b;++i)
#define per(i,a,b) for(int i=a;i>=b;--i)
#define test(t) int t; cin>>t; while(t--) solve()
#define endl "\n"
#define p1 cout<<"YES"<<endl
#define p0 cout<<"NO"<<endl
#define fst ios::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)

int main()
{
    double r;
    cin>>r;
    r*=2;
    r*=3.14159265358979;

    cout<<r<<endl;
}