#include<bits/stdc++.h>
#define rep(i,a,b) for(int i=a;i<=(b);++i)
#define per(i,a,b) for(int i=a;i>=(b);--i)
#define mem(a,x) memset(a,x,sizeof(a))
#define pb push_back

#define mk make_pair
using namespace std;
typedef long long ll;
ll gcd(ll a,ll b) { return b?gcd(b,a%b):a;}
const double pi=acos(-1);
void solve()
{
    double r;
    cin>>r;
    double ans=pi*2*r;
    printf("%.6f\n",ans);
}
int main()
{
	//int _;cin>>_;while(_--)
	solve();
}
