#include<iostream>
#include<algorithm>
#include<vector>
#include<map>
#include<cmath>
#include<string>
#include<bitset>
#include <sstream>
#include<queue>
#include<stack>
#include<iomanip>
#include<functional>
#define rep(i,n) for(ll i=0;i<(n);++i)
#define sayyes cout<<"Yes"<<endl;
#define sayno cout<<"No"<<endl;
using namespace std;
typedef long long ll;
typedef pair<int,int>P;
const int mod =1e9+7;

int main()
{
    int n;
    cin>>n;
    cout<<(double)(2.0*n*3.1415);
    
}
