#include<bits/stdc++.h>
using namespace std;
#define FS ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define ll long long int
#define ld long double
#define pb push_back
#define bp __builtin_popcount
#define sz size()
#define ff first
#define ss second
#define vll vector<ll>
#define vbool vector<bool>
#define vpll vector<pair<ll,ll>>
#define pll pair<ll,ll>
#define vllv vector<vector<ll>>
#define setpri(x) cout<<setprecision(x)<<fixed;
#define all(v) v.begin(),v.end()
#define allr(v) v.rbegin(),v.rend()
#define yesr {cout<<"YES"<<endl;return;}
#define nor {cout<<"NO"<<endl;return;}
// getline (std::cin,name);
ll MOD=1e9+7;
ll ceil1(ll n,ll x){if(n%x==0)return n/x; else return (n/x)+1;}
ll gcd(ll a,ll b){return __gcd(a,b);}
ll lcm(ll a,ll b){return (max(a,b)/gcd(a,b))*min(a,b);}
ll pow1(ll n,ll k,ll mm){
    if(k==0)return 1;
    if(k%2==0)return (pow1((n%mm)*(n%mm),k/2,mm))%mm;
    return ((n%mm)*(pow1((n%mm)*(n%mm),k/2,mm)%mm))%mm;
}
ll pow2(ll n,ll k){ll ans=1;
    while(k>0){
        if(k%2==1)ans=ans*n;n=n*n;k/=2;
    }return ans;
}
ll modinv(ll A,ll M){return pow1(A,M-2,M);}
ll fact(ll n){ll s=1; for(ll i=1;i<=n;i++)s*=i; return s;}
ll factmod(ll n,ll mod){ll s=1; for(ll i=1;i<=n;i++){s*=i;s%=mod;} return s;}
vll fact1tonmod(ll n,ll mod){vll  ans(n+1);ans[0]=ans[1]=1;
    for(ll i=2;i<=n;i++)ans[i]=(ans[i-1]*i)%mod;
    return ans;
}
ll ncrmod(ll n,ll r,ll mod){
    ll ans=((((factmod(n,mod)%mod)*(modinv(factmod(r,mod),mod)%mod))%mod)*(modinv(factmod(n-r,mod),mod)))%mod;
    return ans;
}
ll ncr(ll n,ll r){if(n==1 || r==0)return 1;if(r>n/2)r=n-r;
    ll ans=1;for(ll i=1;i<=r;i++){ans=((ans * n )/i);n--;}return ans;
}
bool isprime(ll n){if(n==1 || (n!=2 && n%2==0))return false; if(n==2)return true; 
        for(ll i=3;i<=sqrt(n);i+=2)if(n%i==0)return false; return true; }
bool func(pair<ll,ll> &a,pair<ll,ll> &b ){
    if(a.ff != b.ff)return a.ff < b.ff;
    return a.ss > b.ss;
}
void solve(){
    ll i,j,l,m,x,y,z,r;
    cin>>n;
  double ans=2.0*3.1415927*(double)n;
  cout<<fixed<<setpricision(10)<<ans;
}
int main(){
    // FS;
    ll i,j,k,n,m,x,y,z,q;
    q=1;
	// cin>>q;
    while(q--){
        solve();
    }
    return 0;
}
