#include <bits/stdc++.h>
#define ll signed long long int
#define FAST ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define MAXX 1000005
using namespace std;

ll a[MAXX], b[MAXX], c[MAXX], k = 0;

signed main()
{
	FAST;
	ll n, m, f = 0, pi = 3.14;
	cin >> n;
	cout << 2 * pi * n;
	
}