#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
#define rep(i, n) for(int i = 0; i < n; i++)
#define all(x) x.begin(), x.end()
#define fi first
#define se second
#define pb push_back
template<class T> inline int chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline int chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
const int INF = 1<<29;
const ll LINF = 1LL<<58;
const int mod = 1000000007;

void solve() {
    double r;
    cin >> r;
    cout << fixed << setprecision(10) << 3.141592 * 2 * r << endl;
    return ;
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    solve();
    return 0;
}
