--description--

An infinite sequence of real numbers $a(n)$ is defined for all integers $n$ as follows:

$$ a(n) = \begin{cases} 1 & n < 0 \\ \displaystyle \sum_{i = 1}^{\infty} \frac{a(n - 1)}{i!} & n \ge 0 \end{cases} $$

For example,

$$\begin{align} & a(0) = \frac{1}{1!} + \frac{1}{2!} + \frac{1}{3!} + \ldots = e − 1 \\ & a(1) = \frac{e − 1}{1!} + \frac{1}{2!} + \frac{1}{3!} + \ldots = 2e − 3 \\ & a(2) = \frac{2e − 3}{1!} + \frac{e − 1}{2!} + \frac{1}{3!} + \ldots = \frac{7}{2} e − 6 \end{align}$$

with $e = 2.7182818\ldots$ being Euler's constant.

It can be shown that $a(n)$ is of the form $\displaystyle\frac{A(n)e + B(n)}{n!}$ for integers $A(n)$ and $B(n)$.

For example $\displaystyle a(10) = \frac{328161643e − 652694486}{10!}$.

Find $A({10}^9)$ + $B({10}^9)$ and give your answer $\bmod 77\,777\,777$.

--hints--

eulersNumber() should return 15955822.

assert.strictEqual(eulersNumber(), 15955822);

--seed--

--seed-contents--

function eulersNumber() {

  return true;
}

eulersNumber();

--solutions--

// solution required