#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define repl(i, l, r) for (ll i = (l); i < (r); i++)
#define per(i, n) for (ll i = n - 1; i >= 0; i--)
#define lper(i, r, l) for (ll i = r - 1; i >= l; i--)
#define fi first
#define se second
#define all(x) (x).begin(), (x).end()
#define CST(x) cout << fixed << setprecision(x)
using ll = long long;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using pl = pair<ll, ll>;
const ll MOD = 1000000007;
const int inf = 1e9 + 10;
const ll INF = 4e18;
const ll dy[4] = {1, 0, -1, 0};
const ll dx[4] = {0, -1, 0, 1};
int main() {
	cin.tie(0);
	cout.tie(0);
	ios::sync_with_stdio(false);
	double r;
	cin >> r;
	CST(10) << r * 2 * M_PI << endl;
	return 0;
}