#include<bits/stdc++.h>
#define pii pair<int,int>
#define x first
#define y second
#define mp make_pair
#define pb push_back
#define ll long long
#define sz size()
#define ft front
#define pu push
#define ty empty
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
const int m5=100042;
int main(void)
{
	const float pie=3.1415926;
	float r;
	cin>>r;
	cout<<(r*2*pie);
	return 0;
    return 0;
}

