# Standard deviation of a list

## Question

Given a list of numbers, write a function to read in the list and calculate the standard deviation. Avoid using any built in library to calculate.

Examples:

```
std_dev([2, 4, 6, 8]) = 2.58199
std_dev([1]) = 'NaN'
```

Given a list of numbers, write a function to read in the list and calculate the standard deviation. Avoid using any built in library to calculate.

Examples:

```
std_dev([2, 4, 6, 8]) = 2.58199
std_dev([1]) = 'NaN'
```