Create a function that sums two arguments together. If only one argument is provided, then return a function that expects one argument and returns the sum.

For example, `addTogether(2, 3)`

should return `5`

, and `addTogether(2)`

should return a function.

Calling this returned function with a single argument will then return the sum:

`var sumTwoAnd = addTogether(2);`

`sumTwoAnd(3)`

returns `5`

.

If either argument isn't a valid number, return undefined.

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