/*starting with the name of almighty ALLAH*/
/*solved by RAKIB JOY*/
#include<bits/stdc++.h>
#define ll long long int
#define pb push_back
#define vc vector
#define vi vc<int>
#define vl vc<ll>
#define dbg(x) cout<<x<<endl;
#define mp(x,y) make_pair(x,y)
#define yes dbg("YES")
#define no dbg("NO")
#define tst int t;cin>>t;while(t--)
#define srt(v) sort(v.begin(),v.end());
#define rsrt(v) sort(v.begin(),v.end(),greater<int>());
#define rj ios::sync_with_stdio(false);cin.tie(0);
#define rvs(v) reverse(v.begin(),v.end());
#define F first
#define S second
using namespace std;
int main()
{
    rj
    double a,b=3.14159;
    cin>>a;
   printf("%.2lf\n",2*a*b);

    return 0;
}
