#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define vi vector<int>
#define vll vector<ll>
#define all(a) a.begin(),a.end()
#define mp(a,b) make_pair(a,b)
#define pb push_back
#define mod 1000000007
#define ft cout<<"for test"<<endl;
#define pi 2*acos(0.0)
int main()
{
    ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
    double r;
    cin>>r;
    cout<<2*pi*r<<endl;

}
