#include <bits/stdc++.h>
using namespace std;
using ld = long double;
#define ldout fixed << setprecision(40) 
int main(void) {
    cin.tie(0); ios::sync_with_stdio(false);
    ld n; cin >> n;
    cout << ldout << 2.0l * sqrtl(n * M_PI) << endl;
    return 0;
}