#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define mp make_pair
#define f first
#define s second
#define ll long long
#define all(_v) _v.begin(), _v.end()

double n;
int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
    cin >> n;
    cout << 2 * n * acos(-1);
    return 0;
}
