#include<bits/stdc++.h>
#define pb push_back
#define FAST ios::sync_with_stdio(false);cin.tie(NULL),cout.tie(NULL);
typedef long long ll;
#define pi acos(-1.0)
using namespace std;

int main()
{
    long double r;
    cin>>r;
    cout<<2*pi*r<<endl;
    return 0;
}
