#include<bits/stdc++.h>
using namespace std; 
int main()
{
    #ifdef LOCAL
        freopen("input.txt", "r", stdin);
        freopen("output.txt", "w", stdout);
    #endif
	int r;
	cin >> r;
	double ans = (2*r*22)/7;
	cout << ans << endl;
	return 0;
}