//AnkitCode99 here....
#include<bits/stdc++.h>
#define endl "\n"
typedef long long int ll;
#define MOD 1000000007
#define mp make_pair
#define pll pair<ll,ll>
#define pb push_back
//max xor btw range of two numbers..
#define max_XOR(a,b) (1 << int(log2(a ^ b) + 1)) - 1
#define vl vector<ll>
#define inf ll(1e18)
#define all(v) v.begin(),v.end()
#define fast ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define rep(i,a,b) for(long long int i=a;i<b;i++)
#define nrep(i,a,b) for(long long int i=a;i>=b;i--)
#define ceil(a,b) a/b+((a%b ==0 )?0:1)
using namespace std;

const ll sz=100005;
const ll szz=1000006;

int main()
{
    fast;
    // #ifndef ONLINE_JUDGE
    // freopen("input.txt","r",stdin);
    // freopen("op.txt","w",stdout);
    // #endif

    ll r;
    cin>>r;
    cout<<2*3.14*r;
    
}//Goodbye...