#include<bits/stdc++.h>
using namespace std;
#define For(i,m,n) for(int i=m;i<n;i++)
#define FOR(i,n) for(int i=0;i<n;i++)
#define ll long long
#define ull unsigned long long
#define lli long long int
#define pb push_back
# define M_PI           3.14159265358979323846  /* pi */
ll mob=1000000007;
using namespace std;
int main()
{ 

int r;cin>>r;
cout<<fixed<<setprecision(20);
cout<<(double)2*M_PI*r;


return 0;
}









