Solve the programming task below in a Python markdown code block.
Scientists working internationally use metric units almost exclusively. Unless that is, they wish to crash multimillion dollars worth of equipment on Mars.

Your task is to write a simple function that takes a number of meters, and outputs it using metric prefixes.

In practice, meters are only measured in "mm" (thousandths of a meter), "cm" (hundredths of a meter), "m" (meters) and "km" (kilometers, or clicks for the US military).

For this exercise we just want units bigger than a meter, from meters up to yottameters, excluding decameters and hectometers.

All values passed in will be positive integers.
e.g.

```python
meters(5);
// returns "5m"

meters(51500);
// returns "51.5km"

meters(5000000);
// returns "5Mm"
```

See http://en.wikipedia.org/wiki/SI_prefix for a full list of prefixes
Also feel free to reuse/extend the following starter code:
```python
def meters(x):
```