#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
const ll MOD = 1e9+7;
const ll INF = 1e18;
#define rep(i,m,n) for(int i = (m); i <= (n); i++)
#define zep(i,m,n) for(int i = (m); i < (n); i++)
#define rrep(i,m,n) for(int i = (m); i >= (n); i--)
#define print(x) cout << (x) << endl;
#define printa(x,m,n) for(int i = (m); i <= n; i++){cout << (x[i]) << " ";} cout<<endl;

int main(){
	cin.tie(0); ios::sync_with_stdio(false);
	
	ll r; cin >> r;
	cout << fixed << setprecision(20) << 2*r*3.141592653 << endl;
	return 0;
}