#include <bits/stdc++.h>
using namespace std;
typedef  long long int ll;
#define pb push_back
typedef long double ld;
#define fi first
#define se second
#define mp make_pair

vector<vector<pair<ll,ll>>> out(1e5);
vector<vector<ll>> in(1e5);
ll inf=1e18;
ll p=1e9+7;
ll phi(ll n) {
    ll result = n;
    for (ll i = 2; i * i <= n; i++) {
        if(n % i == 0) {
            while(n % i == 0)
                n /= i;
            result -= result / i;
        }
    }
    if(n > 1)
        result -= result / n;
    return result;
}
ll power(ll x, ll y, ll p)
{
    ll res = 1;
    x = x % p;
   while (y > 0)
    {
        if (y & 1)
            res = (res*x) % p;
        y = y>>1;
        x = (x*x) % p;
    }
    return res;
}
ll modInverse(ll n, ll p)
{
    return power(n, p-2, p);
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
	ll n,i,j,t,a,b,temp,m,k,x,y,ctr;
	ld r;
	cin>>r;
	ld ans=(2.0)*(3.14159)*r;
	cout<<fixed<<setprecision(6)<<ans;

	return 0;
	
	
}