#include <bits/stdc++.h>
#define _GLIBCXX_DEBUG
#define rep(i,n) for(int i=0;i<(n);++i)
#define dunk(a) cout << (a) << endl
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()
using namespace std;
typedef long long ll;

int main() {
	double r; cin >> r;
	double ans = 2 * r * 3.14159;
	cout << ans << fixed << setprecision(10);
	return 0;
}
