#include <bits/stdc++.h>
#define maxn 200001
#define maxa 2000005
#define MOD  998244353 //1000000007
#define INF 1000000000000000000

using namespace std;

#define int int64_t
#define ll long long
#define pii pair<ll, ll>
#define piii pair<pii, ll>
#define ft first
#define sd second
#define pb push_back
#define rep(i, n) for(ll i = 0; i < n; i++)
#define repr(i, n) for(ll i = n-1; i >= 0; i--)
#define mem(a, b) memset(a, b, sizeof(a))
#define all(v) v.begin(), v.end()
#define edge(v, x, y) v[x].pb(y); v[y].pb(x);
#define popcount __builtin_popcount

void fast(){
    ios_base::sync_with_stdio(false);
    cin.tie(0);cout << setprecision(10) << fixed;
}
ll ceil(ll a, ll b){ return (a+ b-1)/b; }
void flaged(bool flag){
    if(flag) cout << "Yes\n";
    else cout << "No\n";
}
// power and modulo inverse
ll power(ll a, ll b = MOD-2){
    if(b == 0) return 1;
    ll r = power(a, b/2);
    r = (r*r)%MOD;
    if(b%2 == 1) r = (r*a)%MOD;
    return r;
}
// factorial
ll fact[maxa], finv[maxa];
void calf(){
    fact[0] = finv[0] = 1;
    for(ll i = 1; i < maxa; i++){fact[i] = (fact[i-1]*i) %MOD;finv[i] = (finv[i-1]*power(i))%MOD;}
}
// nCr
ll ncr(ll N, ll R){ if(R > N) return 0; return (fact[N]*((finv[R]*finv[N-R])%MOD))%MOD;}



void solve(){
    double n;cin >> n; cout << 2*3.14*n;
}

main(){
    ll t = 1; //cin >> t;
    while(t--){
        solve();
    }
}
