#include <bits/stdc++.h>

#define rep(i, n) for ( int i = 0; i < (n); ++i )
using namespace std;
using ll =  long long;
using P = pair<ll, ll>;

int main( void )
{
	ll r;

	cin >> r;

	cout << (2 * r *3.1415926535) << endl;

	return 0;
}
