#include <bits/stdc++.h>
 
using namespace std;
typedef long long int lli;
#define fast ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
typedef pair <lli,lli> pii;
#define ff first
#define ss second
#define mod 1000000007
#define pb push_back
#define EPS 1e-6

int main()
{
    fast;
    double r;
    cin>>r;
    cout<<fixed<<setprecision(20)<<(2*(M_PI)*r);
    
    return 0;
}