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

typedef long long ll;
#define optimize() ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define file() freopen("input.txt","r",stdin);freopen("output.txt","w",stdout);
#define tt int t; cin>>t; while(t--)
#define nl cout<<"\n";
#define sp cout<<" ";
#define rep(i, a, b) for(long long i=a; i<b; i++)


int main()
{
    optimize();
    long double r, c;
    cin>> r;
    cout<< fixed<< setprecision(20);
    cout<< 2*r*3.1416;
    nl;


    return 0;
}

