#include <bits/stdc++.h>
using namespace std;
#define fo(i,n) for(int i = 0; i < n; i++)
#define pb push_back
#define all(c) c.begin(),c.end()
 
 
int main()
{
 
    int r;
    cin >> r;
    cout << 3.14159 * 2 *r;
 
 
 
 
    return 0;
}