Compute the **n**<sup>th</sup> term of a [series](<https://en.wikipedia.org/wiki/Series (mathematics)>), i.e. the sum of the **n** first terms of the corresponding [sequence](https://en.wikipedia.org/wiki/sequence). Informally this value, or its limit when **n** tends to infinity, is also called the *sum of the series*, thus the title of this task. For this task, use: $S*n = \\sum*{k=1}^n \\frac{1}{k^2}$ and compute $S\_{1000}$ This approximates the [zeta function](<https://en.wikipedia.org/wiki/Riemann zeta function>) for S=2, whose exact value $\\zeta(2) = {\\pi^2\\over 6}$ is the solution of the [Basel problem](<https://en.wikipedia.org/wiki/Basel problem>).
# --instructions--
Write a function that take $a$ and $b$ as parameters and returns the sum of $a^{th}$ to $b^{th}$ members of the sequence.
# --hints--
`sum` should be a function.
```js
assert(typeof sum == 'function');
```
`sum(1, 100)` should return a number.
```js
assert(typeof sum(1, 100) == 'number');
```
`sum(1, 100)` should return `1.6349839001848923`.
```js
assert.equal(sum(1, 100), 1.6349839001848923);
```
`sum(33, 46)` should return `0.009262256361481223`.
```js
assert.equal(sum(33, 46), 0.009262256361481223);
```
`sum(21, 213)` should return `0.044086990748706555`.
```js
assert.equal(sum(21, 213), 0.044086990748706555);
```
`sum(11, 111)` should return `0.08619778593108679`.
