
#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

void solve() {
	int r;	cin >> r;
	cout << 2*3.14159*r << endl;
}

int main() {
	int t = 1;
	//cin >> t;
	while(t--) {
		solve();
	}
	return 0;
}
