##### Canadian Computing Competition: 2009 Stage 2, Day 1, Problem 2

On the way to dinner, the CCC competitors are lining up for their delicious curly fries. The competitors have lined up single-file to enter the cafeteria.

Doctor V, who runs the CCC, realized at the last minute that programmers simply hate standing in line next to programmers who use a different language. Thankfully, only two languages are allowed at the CCC: Gnold and Helpfile. Furthermore, the competitors have decided that they will only enter the cafeteria if they are in a group of at least competitors.

Doctor V decided to iterate the following scheme:

- He will find a group of or more competitors who use the same language standing next to each other in line and send them to dinner.
- The remaining competitors will close the gap, potentially putting similar-language competitors together.

So Doctor V recorded the sequence of competitors for you. Can all the competitors dine? If so, what is the minimum number of groups of competitors to be sent to dinner?

Note: Test cases worth of the points have . Out of these, on test cases worth one third of the points ( of the total points), .

#### Input Specification

The first line contains two integers and .

The second line contains characters that are the sequence of competitors in line (`H`

represents Helpfile, `G`

represents Gnold)

#### Output Specification

Output, on one line, the single number that is the minimum number of groups that are
formed for dinner. If not all programmers can dine, output `-1`

.

#### Sample Input

```
7 2
GHHGHHG
```

#### Description of Sample Input

There are seven competitors: a Gnold programmer followed by two Helpfile programmers, followed by another Gnold programmer, followed by another two Helpfile programmers followed by a final Gnold programmer. Programmers want to go to dinner in pairs.

#### Output for Sample Input

`3`

#### Description of Output for Sample Input

First send the first pair of `H`

s to dinner, leaving `GGHHG`

. Then send the second pair of
`H`

s to dinner, leaving `GGG`

; finally, send in the group of `G`

s. It might be coincidental that
the two pairs of Helpfile programmers entered the cafeteria successively.

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