#include "bits/stdc++.h"
using namespace std;
#ifdef LOCAL
#define debug(x)   cerr<<#x<<" = "<<x<<endl
#else
#define debug(x) 
#endif
#define IOS ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define input_from_file freopen("input.txt","r",stdin)
#define output_to_file freopen("output.txt" , "w",stdout)
#define pii pair<int,int>
#define endl '\n'
#define pb push_back
#define fi first
#define se second 
#define mp make_pair
#define int long long
const int inf = 1e18 + 5;
signed main()
{
    #ifdef LOCAL
        input_from_file;
        output_to_file;
    #endif 
    IOS
    int r; cin>>r;
    double pi = 3.14159265359;
    double ans = 2*pi*r;
    cout<<fixed<<setprecision(20)<<ans<<endl;
    return 0;
}