#include <bits/stdc++.h>
#define db(x) cerr << __LINE__ << ": " << #x << " = " << (x) << endl;
#define ll long long
#define ld long double
#define pll pair<ll, ll>
#define MOD 1000000007
#define FOR(i, n) for(ll i = 0; i < n; i++)
using namespace std;
const long long INF = 3*1e18;

int main() {
    ios::sync_with_stdio(0);    
    cin.tie(0);
    ll r;
    cin >> r;
    double ans = 2 * 3.1415926 * r;
    cout << ans << "\n";
}