#include <bits/stdc++.h>

#define int long long
#define fastio ios::sync_with_stdio(0); cin.tie(0); cout.tie(0)

const unsigned int mod = 1e9 + 7;

using namespace std;

int32_t main() {
	fastio;

	int r;
	cin >> r;
	cout << 2.0 * 3.14 * r;

	return 0;
}
